Selected Correspondence of Descartes
René Descartes

Copyright © Jonathan Bennett 2017. All rights reserved[Brackets] enclose editorial explanations. Small ·dots· enclose material that has been added, but can be read as
though it were part of the original text. Occasional •bullets, and also indenting of passages that are not quotations,

are meant as aids to grasping the structure of a sentence or a thought. Every four-point ellipsis . . . . indicatesthe omission of a brief passage that has no philosophical interest, or that seems to present more difficulty than
it is worth. (Where a letter opens with civilities and/or remarks about the postal system, the omission of thismaterial is not marked by ellipses.) Longer omissions are reported between brackets in normal-sized type. —The
letters between Descartes and Princess Elisabeth of Bohemia, omitted here, are presented elsewhere on thiswebsite (but see note on page 181).—This version is greatly indebted to CSMK [see Glossary] both for a good English
translation to work from and for many explanatory notes, though most come from AT [see Glossary].—Descartes
usually refers to others by title (‘M.’ for ‘Monsieur’ or ‘Abbé’ or ‘Reverend Father’ etc.); the present version omits
most of these.—Although the material is selected mainly for its bearing on Descartes as a philosopher, glimpses
are given of the colour and flavour of other sides of his life.
First launched: April 2013

Correspondence

René Descartes

Contents
Letters written in 1619–1637
1
to Beeckman, 26.iii.1619 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
to Beeckman on 23.iv.1619 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
to Beeckman, 29.iv.1619 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
to Gibieuf, 18.vii.1629 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
to Mersenne, 8.x.1629 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
to Mersenne, 13.xi.1629 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5
to Mersenne, 20.xi.1629 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
6
to Mersenne, 18.xii.1629 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8to Mersenne, i.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
to Mersenne, 25.ii.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
to Mersenne, 18.iii.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
to Mersenne, 15.iv.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
to Mersenne, 6.v.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
to Mersenne, 27.v.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
to Beeckmann, 17.10.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
to Mersenne, 25.xi.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
to Mersenne, 23.xii.1630 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
to Balzac, 15.iv.1631 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
to Balzac, 5.v.1631 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
to Villebressieu, summer 1631 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
to Mersenne, x or xi 1631 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
to Golius, 2.ii.1632 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
to Mersenne, 5.iv.1632 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
to Mersenne, 10.v.1632 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
to Mersenne, vi.1632 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
to Mersenne, xi or xii 1632 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
to Mersenne, late xi.1633 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
to Mersenne, ii.1634 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
to Mersenne, iv.1634 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
to Mersenne, 14.viii.1634 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
to Beeckman, 22.viii.1634 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Correspondence

René Descartes

to Morin, ix or x 1634 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
to Golius, 16.iv.1635 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
to Golius, 19.v.1635 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
to Mersenne, vi or vii 1635 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
to Huygens, 1.xi.1635: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
from Huygens, 3.xii.35 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
to Mersenne, iii.1636 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
from Huygens, 5.i.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
to Mersenne, 27.ii.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
to Huygens, 3.iii.37 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
to Silhon, v.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Fermat to Mersenne, iv or v 1637: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
to Huygens, 20.v.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
to Mersenne, late v.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
to ***, late v.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
to Mersenne, 25.v.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
to Noël, 14.vi.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
to Huygens, 12.vi.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
to Plempius, 3.x.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
to Plempius for Fromondus, 3.x.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
to Huygens, 5.x.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
to Mersenne, 5.x.1637: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
to Noël, x.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
to Huygens, 4.xii.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
to Plempius, 20,xii.1637: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
to Mersenne, end of xii.1637 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Correspondence

René Descartes

Letters written in 1638–1640
57
to Mersenne, i.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
from Pollot to Reneri for Descartes, ii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
to Plempius, 15.ii.1638: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
from Morin, 22.ii.38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
to Vatier, 22.ii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
against Fermat, 1.iii.38 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
to Mersenne, 1.iii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
to Reneri for Pollot, iv or v 1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
to Huygens, 9.iii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
from Mersenne, 28.iv.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
to Mersenne, 27.v.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
to Morin, 13.vii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
to Mersenne, 13.vii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
to Mersenne, 27.vii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
to Hogelande, viii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
to Morin, 12.ix.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
to Debeaune, 12.ix.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
to Mersenne, 11.x.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
to Mersenne, 15.xi.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
to Mersenne, xii.1638 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
to Mersenne, 9.i.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
to Mersenne, 9.ii.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
to Mersenne, 20.ii.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
from Regius, 9.iii.39 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
to Debeaune, 30.iv.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
to Desargues, 19.vi.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
to Mersenne, 19.vi.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
to Mersenne, 27.viii.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
to Mersenne, 16.x.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
to Mersenne, 13.xi.1639 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
to Mersenne, 25.xii.1639: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
to Mersenne, 29.i.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
to Meysonnier, 29.i.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Correspondence

René Descartes

to Hogelande, 8.ii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
to Mersenne, 11.iii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
to Mersenne, 1.iv.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
to Regius, 24.v.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
to Mersenne, 11.vi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
to Mersenne, 30.vii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
to Huygens, 31.vi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
to Mersenne, 6.vi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
to Mersenne, 30.ix.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
to Mersenne, 28.x.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
to Mersenne, 11.xi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
to Gibieuf, 11.xi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
to Mersenne, 11.0xi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
to Colvius, xi.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
to Mersenne, 3.xii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
to Mersenne, xii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
to Mersenne, 24.xii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
to Charlet, xii.40 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
to Mersenne, 31.xii.1640 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

Correspondence

René Descartes

Letters written in 1641–1644
122
to Pollot, mid-i.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
to Mersenne, 21.i.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
to Mersenne for Hobbes, 21.1.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
to Mersenne, 28.i.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
to Mersenne, 4.iii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
to Mersenne, 18.iii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
to Mersenne, 31.iii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
to Mersenne for Hobbes, 21.iv.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
to Mersenne, 21.iv.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
to Regius, v.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
to Regius, v.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
to Mersenne, 16.vi.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
to Mersenne, 23.vi.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
to Mersenne, vii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
to Mersenne, 22.vii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
to DeLaunay, 22.vii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
to Hyperaspistes, viii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
to Mersenne, ix.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
to Mersenne, 17.xi.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
to Regius, xii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
to Regius, xii.1641 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
to Gibieuf, 19.i.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145
to Mersenne, 19.i.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
to Regius, i.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
to Huygens, 31.i.1642: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
to Regius, late ii.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
to Mersenne, iii.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
to Huygens, 26.iv.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
to Regius, vi.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
to Pollot, 6.x.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
to Huygens, 10.x.1642 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
to Picot, 2.iii.1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
to Colvius, 23.iv.1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Correspondence

René Descartes

to Mersenne, 26.iv.1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
to Huygens, 24.v.1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
to Voetius, v.1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
to Vorstius, 19.vi.1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162
to Buitendijk, 1643? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
to Father ****, 1643 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
to Mesland, 2.v.1644 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166
to Grandamy, 2.v.1644 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
to ***, 1644 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
to Charlet, viii.1644 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
to Bourdin, x.1644 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171

Correspondence

René Descartes

Letters written in 1645–1650
172
to Charlet, 9.ii.1645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
to Mesland, 9.ii.1645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
to Mesland, v.1645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
to * * * * , vi.1645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 177
to Regius, vii.1645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178
to Cavendish, x.1645 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
to Mesland, 1645 or 1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
to * * * , iii.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
to Mersenne, 20.iv.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
to Chanut, 15.vi.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183
To Clerselier, vi or vii 1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184
to Mersenne, 5.x.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
to Chanut, 1.xi.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
to Mersenne, 23.xi.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188
to Cavendish, 23.xi.1646 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189
to Chanut, 1.ii.1647 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
to the Curators of Leiden University, 4.v.1647 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
to Chanut, 6.vi.1647 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198
to Queen Christina, 20.xi.1647 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
to Chanut, 20.xi.1647 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203
to Mersenne, 13.xii.1647 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
to Cavendish iii or iv 1648: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
for Arnauld, 4.vi.1648 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
for Arnauld, 29.vii.1648 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
to Pollot, 1648: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
to More, 5.ii.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212
to Chanut, 26.ii.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
to Chanut, 31.iii.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 218
to More, 15.iv.1649: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219
to Clerselier, 23.iv.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222
to Freinhemius, vi.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
to Carcavi, 11.vi.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224
to Carcavi, 17.viii.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224

Correspondence

René Descartes

to More, viii.1649 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
to Brégy, 15.i.1650 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

225
226

Correspondence

René Descartes

Glossary
accident: Often used to mean ‘non-essential property’: yourbeing more than 5’ tall is an accident of you, whereassome philosophers would say that your having the powerof thought is not. But quite often ‘accident’ is used justto mean ‘property or quality’, with no special emphasis on
non-essentialness.

beg the question: Until fairly recently, to ‘beg the question’was to offer a ‘proof’ of P from premises that include P. It
now means ‘raise the question’. It seems that complacently
illiterate journalists (of whom there are many) encountered
the phrase, liked it, guessed at its meaning, and saw noreason to check on the guess.

a priori , a posteriori : In Descartes’s day these phrases
were used to mark the difference between •seeing something
happen and working out what will follow from it and •seeing
something happen and working out what must have caused
it, i.e. between •causally arguing forward and •causallyarguing backwards; quite unlike Kant’s use of the terms
to mean •‘independently of experience’ and •‘on the basis ofexperience’.

burning mirror: A concave mirror which can reflect thesuns ray to a point, creating enough heat there to start a
fire.

animal spirits: This stuff was supposed to be even more
finely divided than air, able to move extremely fast and seep
into tiny crevices. Descartes describes their formation on
page 163.—Apparently some people thought of spirits as so
rarefied as to be almost mind-like(!), and thus suitable tomediate between mind and body; but Descartes is innocentof this absurdity. Its most famous occurrence is in Donne’s
superb lines: ‘As our blood labours to beget / Spirits as likesouls as it can, / Because such fingers need to knit / The
subtle knot that makes us man. . . ’.
art: Any human activity that involves techniques or rules ofprocedure.
AT: This refers either to Œuvres de Descartes, edited by
Charles Adam and Paul Tannery, or to Adam and Tannery
themselves.

catoptrics: The part of optics that deals with reflections.
chimera: A chimera can be a fabulous beast or monster, ora thought or idea of image of something fantastic, fabulous,
etc. In Descartes’s usage it is always the second meaningthat is at work.
circular: Descartes holds that all motion is in a closed loop
(despite his always calling it ‘circular’, he has no views about
its shape). His reason for the loop thesis is this: Absolutely
all space is full of extended substance(s), there are no gaps;
and no material substance can shrink, or expand, or spatiallyoverlap another material substance. Therefore, if body b1 is
to move from location L1 , it must shove aside body b2 , whichmust shove aside b3 . . . and so on; so if an infinite chainof movements is to be avoided, somewhere along the way
there must be body bn which is pushed into location L1 , thus
closing the loop. (It has to be instantaneous: L1 mustn’t beempty for a split second between the departure of b1 and thearrival of bn .)common notion: In Descartes’s usage, a ‘common notion’is a really basic elementary logical truth.

Correspondence

René Descartes

common sense: The phrase ‘the common sense’ was thename of a supposed faculty or organ or brain-region where
inputs from the various senses are processed together andunited.
concurrence: God’s concurrence in an event is his goingalong with it, in some (supposed) sense that is weaker than•his outright causing it but stronger than •his merely notpreventing it.
CSMK: This is volume 3 of The Philosophical Works ofDescartes, translated by John Cottingham, Robert Stoothoff,
Dugald Murdoch, and Anthony Kenny.
doctor: Learned man.
efficient cause: This is an Aristotelian technical term. The•formal cause of a coin is its design, the plan according
to which it was made; its •material cause is the stuff it ismade of; its •final cause is its purpose, namely to be usedin commerce; and its •efficient cause is the action of the diein stamping the coin out of a metal sheet. So the efficientcause is what you and I would call, simply, ‘the cause’.
eminently, formally: These are scholastic technical terms
that Descartes adopts for his own purposes. To say thatsomething has (say) intelligence ‘formally’ is just to say that
it is intelligent; to say that it has intelligence ‘eminently’ is to
say that it has intelligence in some higher form that doesn’t
involve its being straightforwardly intelligent. The distinction
comes into play through the doctrine that whatever is present
in an effect is also present in its cause. Obviously somethingcan be caused to be rigid by a cause that isn’t itself rigid; andGod presumably doesn’t straightforwardly have many of thequalities he causes other things to have—he isn’t square or
muddy or (for that matter) given to telling bad jokes. So thedoctrine takes the form ‘Whatever is present in an effect isalso present formally or eminently in its cause. Descartes’s
only explanation of this terminology is to say that ‘x has
Fness eminently’ means ‘x has the power to cause things
to have Fness’, which you’ll notice turns the doctrine into a
triviality.
de volonté : Descartes repeatedly associates rationally loving x with joining oneself de volonté with x. This doesn’tmean joining oneself voluntarily, by volition [volonté]; it isa technical term, which he explains on page 191 where
he equates ‘x joins itself to y de volonté’ with ‘x considers
itself and y as forming two parts of a single whole’. A bit
less abruptly, you join yourself de volonté with the person
you love if you will yourself into a state in which you feel as
though you and that person are the two parts of a single
whole.
ens per accidens, per se: A pyramid is a collection of stone
blocks that constitute an ens per accidens = an entity by
happenstance. It just happens to be the case that they areinter-related in a way that makes them a pyramid, a thing,an ens. They don’t have any features that intrinsically draw
them together, somehow making them belong together as asingle entity; that would be an ens per se.
heaven: Sometimes Descartes uses ‘the heavens’, as we stillsometimes do, to mean ‘the whole visible universe outside
the earth’. But in the Principles of Philosophy and some of
his letters ‘heaven’ occurs as a technical term referring to
any large spherical mass of rotating fluid material with astar or planet at its centre. The earth, he says, ‘is completely
immersed in a very fluid heaven’.
indifferent: A situation where your will is ‘indifferent’ withrespect to your doing A is a situation where you are underno external pressure to do A and none to refrain from doing
A. For finer tuning, see page 175.

Correspondence

René Descartes

ineffable: Too great to be fully described in words. (Theantonym ‘effable’ occurs these days only in jokes.)
inform: When Descartes says that your body is ‘informed’by your soul, he means only that your body has that soul,is united with it in the standard body-soul manner. It’s odd
that he uses this verb in this way: it echoes the Aristotelian
doctrine that your soul is the form of your body; and that
doctrine, whatever it means, is denied by Descartes’s thesisthat your body is one substance and your soul is another.
interpenetration of dimensions: Descartes holds that it
impossible for two distinct •portions of matter to overlap
spatially: for any two such items, the volume of them bothis the sum of the volumes of each separately. For him thisis equivalent to saying that two distinct •regions of space
can’t overlap; and he expresses by saying that he rejects the‘interpenetration of dimensions’.
metempsychosis: The movement of a soul from one bodyto another.
mœurs: A person’s mœurs includes his morality, his basic
habits, his attitudes and expectations about how people will
behave, his ideas about what is decent. . . and so on. This
word—rhyming approximately with ‘worse’—is left untranslated because there’s no good English equivalent to it.
moral certainty: A degree of certainty that is high enoughfor practical purposes, high enough to make practical doubt
unreasonable; similarly with morally impossible. (In thisphrase ‘moral’ is used in its old sense of ‘having to do withhuman behaviour’.)natural light: If you know something to be true just by
thinking hard about it in the right way, Descartes will saythat you know it ‘by the natural light’.

numerical identity: To say that x is numerically identicalwith y means simply that x is y, which is equivalent to sayingthat x and y are one—that’s how ‘numerical(ly)’ comes into it.
Why have any adjective or adverb in these contexts? Because
the writer thinks that the reader might take the unvarnished‘identity’ to refer to some kind of mere similarity.
objective: When Descartes speaks of the ‘objective being’of an idea he is referring to its representative content, thebeing that is its object, the item that it is about.
parhelia: Two bright patches flanking the sun, sometimes
called ‘false suns’.
passion: When Descartes speaks of ‘passions’ that people
and other animals have, he using the word in about thesame sense as we do. Outside the animal context the word is
the antonym of ‘action’: action/passion = doing/undergoing.
Pelagian: Follower of Pelagius, a 4th-century theologian
whose stress on the role of human effort as a means tosalvation was thought by many to push divine grace out ofthe picture.
pineal gland: This is the current name for the glandthat Descartes always refers to as ‘the gland called “theconarium”’.
prejudice: This translates the French préjugé and the Latinpræjudicium. These basically mean ‘something judged or
believed in advance’ (of the present investigation, of theevidence, or of etc.). These days ‘prejudice’ usually has thenarrower meaning of ‘something pre-judged concerning race,
sex, etc.’. To avoid that taint, CSMK uses ‘preconceived
opinion’ (7 syllables); the present text will use ‘prejudice’ (3syllables) accompanied by this warning.

Correspondence

René Descartes

princess: When Descartes speaks of Queen Christina asa princess he is following a usage that used to be fairly
common for ’prince’ (and its cognates in French and Latin),
namely as standing for any ruler of a state, whether a kingor queen or duke or count etc.
principle: In Descartes’s writings a principe (French) or
principium (Latin) is often a certain kind of universal
proposition—e.g. in the title standardly translated as Principles of Philosophy. But he sometimes uses one of thesewords in a sense, once common but now obsolete, in which
it means ‘source’, ‘cause’, ‘driver’, ‘energiser’, or the like (seepages 23 and 215). The English ‘principle’ also had thatsense; Hume’s Enquiry Concerning the Principles of Morals
is, he tells us, an enquiry into the sources in human natureof our moral thinking and feeling.
privation: A privation in x is x’s not having something that
it ought to have. If a person can’t speak, that is a privationin him; a rock’s lack of the ability to speak is not a privationin it but a mere negation.
rarefied: In early modern times, ‘rare’ and the French rare
meant the opposite of ‘dense’, and was usually understoodto mean ‘very finely divided’.
real quality, real accident: These phrases use ‘real’ in its
old sense of ‘thing-like’ (from Latin res = ‘thing’). The core
thought is this: if heat, for example, is a ‘real quality’ or‘real accident’, then any instance of heat can be thoughtof independently of anything’s having it. When a thing x
comes to be hot, what happens is that it comes to havea real quality, a particular instance of heat. Descartes
rejects this, and holds that predicative propositions shouldbe thought of as having the form ‘x is-hot’ rather than
‘x relates-by-possession-to hotness’. When on page 158

Descartes says that he doesn’t credit motion with any more
reality than is generally attributed to shape, he means thatphilosophers generally wouldn’t speak of a ball’s being roundas a result of a thing-like instance of roundness that theball possesses; and he says that the same goes for the ball’s
being in motion.
reflection, refraction: How light bounces off a mirror, howlight tilts as it enters a translucent medium. The problem
with refraction was to get a sound general account of how theangle at which the light meets the surface of the translucent
body [incidence] relates to the angle at which it carries on fromthere [refraction]. This could involve light going from air into
glass or from glass into air; this problem was central to themaking of optical lenses,
reminiscence: Plato’s doctrine that things you know without having learned them from experience or from otherpeople are things you remember from a previous life when
the soul you now have was joined to a different body.
School: The ‘Schools’ were philosophy departments thatwere almost entirely under Aristotle’s influence, as mediatedby Roman Catholic philosophers and theologians.
science: In early modern times the English word ‘science’,
the French science and the Latin scientia applied to any bodyof knowledge or theory that is (perhaps) axiomatised and(certainly) well founded and conceptually highly organised.
sensible: Translating French sensible and Latin sensibilis,
this usually means ‘capable of being sensed’, i.e. ‘. . . of being
perceived through the senses’. But on page 217 and perhaps
elsewhere, Descartes uses ‘sensible quality’ to refer to what
are commonly called the ‘secondary qualities’ such as colour,
smell, sound, etc. and not including shape and size, thoughthese are perceptible by the senses.

Correspondence

René Descartes

soul: This translates âme. It doesn’t obviously mean anything different from esprit = ‘mind’, and has no theologicalimplications.
species: When on page 103 Descartes speaks of ‘the species
that enter the eyes’ etc. he is using the language of a theory ofAristotle’s that he doesn’t actually believe. According to thistheory, when you see a kitten a tiny representation of a kitten
enters your eyes, and this representative something-or-otheris called a ‘sensible species’. All Descartes needs from thison page 103—and presumably all he intends—is to speakof eyesight as involving a something-or-other entering youreyes.
speculative: This means ‘having to do with non-moral
propositions’. Ethics is a ‘practical’ discipline, chemistry
is a ‘speculative’ one.
substantial form: When Descartes first uses this term here,on page 25, it is not clear what he means by it. In manyother places—e.g. on pages 75 and 136—he merely mentions
it as an item in false Aristotelian metaphysics. In his letter
to Regius on January 1642—starting on page 148—he saysthat he isn’t denying that there are substantial forms but
merely saying that he can do (meta)physics without them.
subtle: When Descartes speaks of some matter as ‘subtle’,he means that it is extremely finely divided, more fluid
than water; and he usually thinks of the ultra-tiny particles
composing it as moving very fast.
transubstantiation: The doctrine that in the Eucharist thebread comes to be part of the substance of Christ’s bodyalthough it still has the qualities of mere bread.
violent: Aristotle divided motions into ‘natural’ and ‘violent’:
the movement to the ground of a dropped pebble is natural,
its upward movement when you throw it up is ‘violent’.

Thus when on page 57 Descartes rejects the natural/violent
distinction, he is rejecting Mersenne’s apparent assumptionthat some states of water are natural and others are not(though he would hardly say that the others are ‘violent’).
vivid: This belongs to the pair‘vivid’ and ‘clear’,which translates the Latin
clarus and distinctus
and the Frenchclair and distinct.
Every other English translator has put‘clear’ and ‘distinct’
but this is certainly wrong. The crucial point concerns clarus(and the French clair). The word can mean ‘clear’ in oursense, and when Descartes uses it outside the clarus etdistinctus phrase, it seems usually to be in that sense. But
in that phrase he uses clarus in its other meaning—its morecommon meaning in Latin—of ‘bright’ or ‘vivid’, as in clara
lux = ‘broad daylight’. If in the phrase clarus et distinctus
Descartes meant clarus in its meaning of ‘clear’, then what’s
left for ‘distinctus’ to mean? Descartes’s only explanation ofthese terms is in Principles of Philosophy 1:45–6, a passage
that completely condemns the usual translation. He writes:‘I call a perception claram when it is present and accessible
to the attentive mind—just as we say that we see somethingclare when it is present to the eye’s gaze and stimulates itwith enough strength and accessibility. I call a perception
distinctam if, as well as being clara, it is so sharply separated
from all other perceptions that every part of it is clarum. . . . A
perception can be clara without being distincta but not viceversa. When someone feels an intense pain, his perception of
it is clarissima, but it isn’t always distincta because people
often get this perception muddled with an obscure judgment

Correspondence

René Descartes

they make about something they think exists in the painful
spot. . . .’ and so on. He can’t be saying anything as stupid
as that intense pain is always extremely clear ! His pointis that pain is vivid, up-front, not shady or obscure. And
for an idea to be distincta is for every nook and crannyof it to be vivid, i.e. for it as a whole to be in our sense‘clear’.—Sometimes when clair and distinct occur together,
the traditional translation is forced on us because distinct isused as a relational term rather than a one-place predicate;

there’s an example of this on page 137, where notions are
spoken of as claires and distinctes les unes des autres—clear
and distinct from one another.
we: Sometimes when this version has Descartes speakingof what ‘we’ may do, he has written of what ‘one’ may do. It
is normal idiomatic French to use on = ‘one’ much oftenerthan we can use ‘one’ in English without sounding stilted.
He often slides from on to nous, clearly not intending any
distinction; for example, paragraph (i) on page 66.

Correspondence

René Descartes

1638–1640

Letters written in 1638–1640
to Mersenne, i.1638:

Llens-maker’ whom he has now met and thinks well of; if he gets far[This letter opens with two pages of disparaging remarks
about Fermat. He has a bold and lively mind, Descartes
says, but he has been overpraised by people who aren’t
qualified to judge his work, and this is doing him harm.
Then a renewed request to Mersenne to pass on anything hehears for or against Descartes’s own work. Finally:]
You ask whether I think that water is in its natural state
when it is liquid or when it is ice. I reply that I don’t regard
anything in nature as violent [see Glossary] except in relation
to the human intellect, which calls ‘violent’ anything that
isn’t in accordance with its will or with what it judges ought
to be the case. It’s no less natural for water to be ice when
it is very cold than to be liquid when it is less cold, because
the causes of each are equally natural.
Lenough with his work, Descartes is willing to go to Paris to help him
further, preferring him to Cardinal.]Lasking him to ‘have pity’ on the world by publishing all his work.]from Pollot to Reneri for Descartes, ii.1638:[This version follows CSMK in accepting ‘from Pollot’ and ‘to Reneri’; AT
isn’t sure of either.]I’m not brave enough to put my difficulties over Descartes’s
work directly to him; so I ask you to pass them on to him,
doing this in such a way that he will find them acceptable, as
coming from someone who is more concerned with learning
than with contradicting.
(1) The second of his moral rules ·in Part 2 of the Discourse on the Method · says that if we have decided to act inaccordance with some opinion, we should continue to abide
by it—even if it is highly doubtful—as firmly as if it wereutterly certain. This seems dangerous. If the opinion is false
or bad, the more we follow it the more we’ll be involved witherror or vice.
(2) The third rule is not a philosopher’s resolution but
rather a fiction that someone can use to soothe and deceive
himself ·in times of trouble·. If something is possible but
there are reasons for disregarding it, that is what a philosopher should do—not pretend that they are impossible. And
no-one with common-sense will ever believe that nothing is
in his power except his thoughts.

[i.38: Descartes writes to Mersenne with more criticisms of Fermat,

of Descartes’s Geometry. ][i.38:[2.ii.38: Huygens writes to Descartes, apologising a little for Fromon-dus, deploring the state of the world, praising Descartes to the skies, andand dismissive comments on something Fermat had written in criticism

L[25.1.38: Descartes writes to (probably) Huygens, mainly about ‘your

Plempius writes to Descartes in response, he says, toDescartes’s frequently expressed wish to hear of anything that Plempius
finds wrong with his account of the movement of the heart. He quotes
a passage from Aristotle in support of his claim that ‘your new theory isold’. Then a couple of pages objecting to some of Descartes’s details.]

L[25.1.38: Descartes writes to Mersenne about: •letters that seem tohave gone astray; •a lens-maker named Cardinal who is having somesuccess, but ‘I am not sure whether to want him to follow my designs’
because he likely to get things wrong on his first attempt and ‘then blameme’; •Mydorge, who is so able and worthy that he will in time, Descartes
expects, come around to Descartes’s views about vision; •and a few othersmall things.]

57

Correspondence

René Descartes(3) The first principle of his philosophy is: I think, so
I am. This is no more certain than plenty of others suchas I breathe, so I am or than this: Every action presupposes
existence [i.e. ‘presupposes something that performs the action’]. ‘You
can’t breathe without a body, but you can think withouta body’—well, that ought to be shown through a clear
demonstration. Of course we can imagine •that we don’t
have a body (though it isn’t easy to do), and •that we canlive without breathing; but it doesn’t follow from this that we
don’t have body or that we can live without breathing.
(4) So there’s a need to prove that the soul can think
without the body; Aristotle presupposes it in one of his
axioms, but he doesn’t prove it. He holds that the soul can•act without bodily organs, from which he concludes that itcan •exist without them; but he doesn’t prove the premise,and experience contradicts it. . . .
(5) If we doubt ·the existence of· things in our environment, it doesn’t follow that there is some being more
perfect than we are. Most philosophers have doubted manythings. . . .without concluding that there is a divinity; there
are other proofs that can give one the thought of God andprove his existence.
(6) We see that the beasts make their attitudes and passions known through their sort of language: they have manysigns showing their anger, fear, love, sorrow, regret at having
acted badly. . . . It’s obvious that the behaviour of animals isdriven by a principle [see Glossary] that is more excellent thanmerely being compelled to act by the state of their organs.
The principle I’m talking about is instinct—something thatnever occurs in a machine or in a clock, which don’t havepassions or attitudes as animals have.
(7) The author says ·near the end of Part 5· that the soulmust be necessarily created; he’d have done well to give areason for that.

1638–1640[A note on item (7): The phrase ‘must be necessarily created’ (doit êtrenécessairement crée) is peculiar; and it doesn’t connect with anything in
the Discourse. What Descartes does say in the Discourse is that thehuman soul must be created expressly (doit être expressement crée) ,meaning that whatever created your soul must have created specificallyit rather than something or other out of which your soul arose. Thereis nothing peculiar about that, and Descartes’s reply to (7) on page 69
precisely fits the expressement version and has nothing to do with thenécessairement version. Conjecture: •a slip of the pen on Pollot’s part,and •an editorial interference with what Descartes wrote (we don’t havethe manuscript).](8) If light were extended, like a stick, it would be nota •movement but a •line that pushes. And if it were a
movement ·of something· that goes from the sun to us, itwouldn’t do that in an instant, because all movement takes
time; and if •light has to pass. . . .across an interval full of
bodies that are much bigger than the subtle [see Glossary]matter that carries •it, those bodies will bounce it around sothat it doesn’t go in a straight line.
(9) Given that the author says that he is writing methodically, clearly, and distinctly, you would expect him to showwhat this ‘subtle matter’ that he supposes is. We are entitledto ask:—Does it exist?
—If it does, is it •elementary [i.e. the kind of stuff that ordinary
things are made of] or rather •a kind of ether ·that exists
only in outer space·?
—If it is elementary, is it •an element itself or rather •aningredient in all the elements?
(10) If water is liquid only because this ‘subtle matter’
makes it so, it follows that ice doesn’t melt any faster near
the fire than it does elsewhere. ·This is obviously false, so· it
has to be admitted that what melts ice is not subtle matterbut heat.

58

Correspondence

René Descartes(11) It’s hard to imagine that water is—·i.e. that its tiny
particles are·—shaped like eels. And the reasons given for
this. . . .show only that the parts of water are slippery andable to fit into all sorts of shapes; they don’t show thatthey must be shaped like eels. And if the most penetrating
bodies must be eel-shaped, then air must even more so thanwater is.
(12) If salt has its taste because of its sharp pointed shape,
other bodies with that shape would taste salty, whereas
in fact they are bland. It would also follow that liqueurs[= ‘liquids’, or = ‘alcoholic drinks’, or = ‘liqueurs’], which Descartes
says are eel-shaped with nothing sharp about them, would
be tasteless, especially those that are mild and don’t havethe salt-like sharp point. And, finally, the taste would be
merely external shape, not an internal quality of the salt;
and salt’s power to stop meat from rotting would consist
merely in its pointedness, its shape.
[Some further challenges to Descartes’s explanations in
Meteorology. 13 Why some bodies sink in water and others
float. (14) If small particles of salt are shaped as Descartes
says they are, and are rigid as he also says they are, it shouldbe easy to strain the salt out of sea-water. (15) Then some
remarks about inconsistent spelling in Descartes’s text.]L

had first introduced to him. Having been to Amsterdam and seen a model
this man has made, Descartes is optimistic about his eventual success.
But he needs copper and steel for making his machine, and there iscompetition for those materials. Descartes now asks Huygens to do what
he can to obtain from the Dutch government an octroi for this man—acertificate that would entitle him to priority.]to Plempius, 15.ii.1638:
I had been eagerly awaiting your objections to my views on
the movement of the heart, and have been well rewarded.
When I considered your learning, intelligence and character,not to speak of the kindness you have shown me, I knew thatyour objections would be erudite, ingenious and unsullied
by any prejudice due to ill-will; and I wasn’t wrong in
my judgement. I thank you for •sending them to me and•showing me how to support my views with the authority
of Aristotle. That man was lucky! Anything that he wrote,
whether thoughtfully or casually, is regarded by most people
today as having oracular authority. So there’s nothing moreI could wish for than to be able to follow in his footsteps
in all things without departing from the truth; but on the
point at issue I couldn’t boast of having done as well as that!
It’s true that I say as he did that the beating of the heart
is due to the expansion of liquid heating up within it; butby ‘liquid’ I simply mean blood; I don’t talk as he did of ‘theexpansion of liquid that is being continually produced fromfood, causing the outer membrane of the heart to expand’.
If I said any such thing I could be refuted by many clear
arguments; and if I said that it was only the outer membraneof the heart that swelled, ignoring the ventricles, the bloodvessels and the valves, I would be inviting the suspicion that
I had never actually looked at the structure of any animalheart. Drawing a true conclusion from false premises, it[12.ii.38: Descartes writes to Pollot thanking him for his tokens ofesteem, and saying that he will do anything in his power to repay them.
He had a few separately printed copies of the Geometry to be given to
people skilled in geometry, and he has set aside one for Pollot. He hasn’t
kept a copy of the short course on mechanics that he wrote for Huygens[see page 50], but he’ll be happy for Pollot to see it if Huygens is willing;
but also happy if he isn’t willing, because he (Descartes) is ashamed of
its imperfections.]

L

1638–1640[12.ii.38: Descartes writes to Huygens, reporting on his recent doings

with the tourneur—operator of a lathe for making lenses—whom Huygens

59Correspondence

René Descartes

seems to me, is no better than drawing a false conclusionfrom them. If two people reach the same place, one by theright road and the other by a wrong one, we oughtn’t to thinkthat the former is following in the other’s footsteps.
(1) You object that sometimes even in a heart that has
been taken from the body and dissected, individual partsof it go on beating although no blood is flowing into or outof it. Well, I once made a rather careful observation of thisphenomenon in fish, whose hearts after removal from thebody go on beating for much longer than the heart of any
terrestrial animal. But I could always judge—and in manycases I could see—that some remaining drops of blood had
fallen from higher up into the lower part where the pulse wasoccurring. This easily convinced me that even a tiny drop ofblood falling from one part of the heart into a slightly warmer
part was enough to cause this beat. Bear in mind that the•smaller the quantity of any liquid the •easier it is to rarefy.
The oftener our hands make some movement, the more apt
they are to make it again on future occasions; similarly, as
the heart continually expands and contracts right from thefirst moment of its formation, the slightest force comes to beenough to push it into this repeated movement. . . . Anyway,
this objection strikes me as much more damaging to thecommon view that the heart’s movement is due to somefaculty of the soul. How could the movement of the cut-up
bits of the heart depend on the human soul, when it’s anarticle of faith—·in the Aristotelian philosophy·—that therational soul is indivisible. . . .?
(2) Your second objection is one that Galen made at theend of his book on the question whether blood is contained in
the arteries. I have never performed the relevant experiment:
it would be hard for me to perform, and I don’t think it would
be worthwhile. [There follow two pages of details explaining
why it wouldn’t be worthwhile. Then:]

1638–1640

We aren’t impressed by the authority of Galen when hesays in various places that What happens is not that thearteries expand because of what is in them, but rather thatmatter flows into them because they expand. For this isdisproved by a decisive experiment that I have seen done
several times and did again today in the course of writing
this letter. [Descartes describes at considerable length a
protracted vivisection—cutting open a live rabbit in order tosee how it heart responds to various changes. We can spare
ourselves the details of this. Descartes concludes:] This
experiment is fatal to Harvey’s view about the movement of
the heart, for he clearly states the very opposite, namely thatthe ventricles dilate. . . .in order to take in blood and thencontract. . . .in order to force the blood into the arteries.
(3) You say that if the heart’s dilation is due to therarefaction of the blood, the expanded stage of the heart
should last longer than it in fact does. Perhaps you think
this because you are imagining the rarefaction as being likewhat happens when water is boiled to make steam; but
there are other sorts of rarefaction, e.g. when the characterof the liquid remains the same but its volume increases.
The water-into-steam kind of rarefaction is obviously quite
different from the rarefaction of the blood in the heart [andDescartes goes on to give two reasons for this].
The second sort of rarefaction, where the liquid increases
in volume, can be either gradual or instantaneous. In
the gradual sort the parts of the liquid gradually take onsome new motion or shape or position that causes the gaps
between them to increase in number or size. . . . In the
instantaneous sort of rarefaction . . . .most of the particlesof the liquid, which are randomly dispersed throughout its
volume, undergo some simultaneous change that causes
them to take up significantly greater space. The facts
show that this is how blood is rarefied in the heart, for60

Correspondence

René Descartes

the expansion takes place instantaneously. If we attend to
all the points made in Part 5 of my Discourse on the Method
we should have no more doubts about this than we haveabout whether oil and other liquids are rarefied in this way
when we see them suddenly boiling up in a pot over the fire.
The entire fabric of the heart, the heat in it, and the very
nature of the blood all contribute to this effect; nothing that
we perceive by the senses seems to me more certain than
this. As for the question of heat: we don’t feel much heat infishes, but their hearts do feel warmer than any other organs
in their body. . . .
It remains for me to reply to your objections against thecirculation of the blood.
The first objection is that arterial blood is different fromvenous blood, I pointed this out in my Discourse on the
Method as a possible objection against Harvey, because
he holds that no change in the blood occurs in the heart.
I on the other hand wasn’t threatened by this objection
because I hold that when the blood is in the heart it suddenly
expands—boils, as it were—and it could hardly go throughthat without suddenly changing. . . ,Then there’s the experiment in which most of the veins
going to a limb are tied, while the arteries remain free. When
this is done, you say, the limb doesn’t swell up, but rather
wastes away gradually through lack of nourishment. Surely
two situations have to be distinguished here. (i) When theveins have been ligated as described, they will certainly swella little, and if you open one of them above the ligature nearly
all the blood in the body can flow out, as surgeons see
every day. I think this provides not merely •highly probableevidence but •conclusive proof of the circulation of the blood.
(ii) I can readily believe that you are right about the resultsof leaving the veins ligated for a long time, though I haven’t
investigated this for myself. For if the blood in ligated veins

1638–1640

stagnates, it will soon become quite thick and hardly fit for
nourishing the body; and no fresh blood will reach it fromthe arteries, since the tiny channels between the arteries andthe veins will all be blocked by the thick blood. Perhaps theveins themselves will contract a little, owing to a loss of thefluid content of the blood brought about by imperceptible
evaporation. But this poses no difficulty for the circulation
thesis.
To sum up, even if I regard your objections as the mostpowerful that could be raised against my views on the
movement of the heart and blood, not one of them induces
me to change my view. But please let me know whether you
think that my brief replies really answer your objections.

from Morin, 22.ii.38:
Since I had the honour of meeting you in Paris I have thoughtof you as having a mind that could leave something rare andexcellent to posterity; and I am delighted to see that I wasright, by seeing the fine book that you have published onmathematics and physics. . . . In mathematics you’ll have
only people who admire the scope and elevation of your mind;
but in physics, I don’t think you’ll be surprised that there
are people who contradict you.
[He then presents difficulties and objections—20 pagesof them—that aren’t given here because we learn enoughabout them from Descartes’s long reply on 13.vii.38 startingon page 74. He closes thus:]
I could present various other difficulties concerning various points in your physics, but for the present I will settle
for having you clarify the nature of light for me, if you think
I am worthy of that favour. Mersenne can assure you that I
have always been one of your partisans. By temperament I
really hate the ill-natured mob who, when they see a superior
61

Correspondence

René Descartes

intellect like a new star in the sky, instead of •wishing him
good fortune in his efforts and inventions, •turn on him
enviously and do their best to blot out his name, his glory,
and his merits—despite the fact that his generosity withhis results is drawing them out of their ignorance. I try
to keep at a safe distance from these back-stabbers. Later
generations will pity me for my misfortunes and, speaking of
this hard century, will say: ‘Back then, fortune didn’t favour
the learned.’ I hope it will do better for you than it has forme, so that we can see your new physics, the principles ofwhich will, I’m sure, remove all my difficulties. . . . Of all thelearned men I know you are the one I honour most, for yourvirtue and your big-hearted plans.

1638–1640

I will answer you point by point. I must say first thatmy purpose was not to teach the whole of my method in the
discourse in which I propound it, but only to say enough toshow that the new views in the Optics and the Meteorology
were not casual thoughts and might be worth the troubleof examining. I couldn’t display the use of this method in
the three treatises that I published, because it prescribes
an order of •research that is quite different from the one I
thought proper for •exposition. Still, I gave a brief sample
of it in my account of the rainbow in Meteorology, and if
you reread that rather difficult passage I expect it to satisfy
you more than it did the first time. I attached these threetreatises to the Discourse that precedes them because I’mconvinced that if people examine them carefully and comparethem with earlier writings on the same topics, they’ll see thatthe method I’m using is no ordinary one and is perhaps
better than some others.

to Vatier, 22.ii.1638:
I am overwhelmed by your kindness in studying my book of
essays with such great care, and sending me your opinionof it with so many marks of goodwill. When I sent it to
you I should have enclosed a letter assuring you of my very
humble service, were it not that I was hoping—vainly as
it turned out—to circulate the book anonymously. I mustbelieve that it is your affection for the father rather thanany deserts of the child which has made you welcome it sofavourably. I am extremely grateful to you. Perhaps I am
too flattered by the very favourable things you say in yourtwo letters, but I must say frankly that no-one, among all
those who have been good enough to express an opinion of
my work, has done me such good justice as you. No-one
else’s criticism has been so favourable, so unbiased and sowell informed. By the way, I am surprised that your second
letter followed so closely on your first. I received them moreor less at the same time, though when I saw your first I wassure that I must not expect another before your vacation.

What I wrote in the Discourse on the Method about theexistence of God is indeed too obscure; although it’s themost important section in the book, it is—I admit—the least
worked out. . . . The main reason for that is that I didn’t wantto take the risk of going into detail about the arguments ofthe sceptics, or say everything that is needed to withdraw themind from the senses. To have a proper sense of the certainty
and evidentness of my kind of argument for God’s existence
you need to have a clear recollection of arguments that show
the uncertainty of all our knowledge of material things; andthese thoughts did not seem to me suitable for inclusion
in a book that I wanted to offer something even to womenwhile also giving the finest minds something to think about.
I confess also that this obscurity arises partly—as you rightly
observed—from my assuming that certain notions that the
habit of thought had made familiar and evident to me must
be equally so to everyone; for example the supposition that62

Correspondence

René Descartes

since our ideas can’t get their forms or their being except
from external objects or from ourselves, they can’t represent
any reality or perfection that isn’t either in those objects orin ourselves. I’ll explain this further in a second edition. . . .
I’m grateful for your care in examining my view about the
movement of the heart. If your physician has any objections
to it I’ll be glad to have them and won’t fail to reply. Abouta week ago a friend of mine who is a Professor of Medicine
at Louvain [Plempius] offered seven or eight objections to thatsame material, and I sent him two sheets in reply. I would
like to receive more of the same kind about all the difficulties
that crop up in my attempted explanations. I shan’t fail to
reply carefully to them, and I’m sure I can do this without
offending those who present them. This is something thata group can do more easily than one man on his own, andno-one could do it better than the members of your Society[the Jesuits]. I would regard it as a great honour and favour
if they took the trouble to do this; it would be the quickest
way to find out all the errors or all the truths in my works.
As for light, if you look at the third page of the Optics
you’ll see that I said explicitly that I was going to speak
about it only hypothetically. Indeed, since the treatise that
contains the whole body of my physical theory is named On
Light, and since in it I explain light in greater detail and at
greater length than anything else, I didn’t want to write it
all out again in Optics but only to convey some idea of it bycomparisons and hints, so far as seemed necessary for thelatter work. [Descartes is referring here to a work that was published

1638–1640

are so closely connected and so interdependent that no-one
could steal any one of them without knowing them all.
Please tell me without delay about difficulties you find inwhat I have written on refraction or anything else; because if
you wait until my more detailed views on light are published,that may be a long wait! I can’t prove a priori [see Glossary] theassumptions I made at the start of the Meteorology without
expounding the whole of my physics; but the empiricalresults that I have deduced rigorously from them, and that
can’t be deduced in that way from other principles, seemto me to prove them sufficiently, a posteriori. I realisedthat this procedure would shock the readers at first, andI think I could easily have prevented this by two changes
in my handling of these starting-points: •not calling them‘assumptions’, and •not stating them until I had given somereasons to prove them. I chose the procedure because(a) I thought I could strictly deduce my results from thefirst principles of my metaphysics, so I wanted to ignore
other kinds of proofs; and (b) I wanted to see whether
the simple exposition of truth, without any quarrels with
contrary opinions, would carry conviction. Those of myfriends who have read most carefully my treatises on Optics
and Meteorology assure me that I have succeeded in this. At
first they found the material as difficult as everyone else did;but now that they have reread it three or four times, they tellme, they no longer find anything it that they think is open
to question. And it isn’t always necessary to have a priori
reasons to convince people of a truth. Thales—or whoever
it was who first said that the moon receives its light fromthe sun—presumably had no support for this except thatit provides an easy explanation for the different phases ofthe moon; that was enough to ensure that from then until
now this view has been peacefully accepted by everyone. My
thoughts are so interconnected that I look forward to the time

posthumously under the title The World, or Treatise on Light.]

You express pleasure at my not allowing others to getin first in publishing my thoughts; thank you for that. But
that’s something I have never been afraid of. •It matters little
to me whether I am the first or the last to write what I write,
provided that what I write is true. •Anyway, all my thoughts
63

Correspondence

René Descartes

1638–1640

when people will find my principles—having become familiarby frequent study, and being considered all together—are aswell proved by the consequences I derive from them as theborrowed nature of the moon’s light is proved by its waxing
and waning.

from the home of my masters, towards whom I will never failin gratitude.

[In the background of this paragraph is the fact that Descartes[This was presumably directed to Roberval and Étienne Pascal.] I am

would have published the works in question if he hadn’t been scaredsurprised that the treatise Maxima and Minima—which wassent to me a while ago and which I now learn is Fermat’s
work—has its defenders. It seems to me that they have no
success in excusing it.
[Descartes now embarks on several pages of geometry,specifically rules for determining tangents to various curves:
Fermat’s rules are wrong, he says, and the pro-Fermat
writers have misrepresented Descartes’s work in order tocriticise it. At the end of all that:]
As for other things that these gentlemen—·Fermat’s
defenders·—say that he has discovered, I would like tobelieve whatever they say. But I have never seen anything by
him except •this Maxima and Minima and •a copy of a letterin which he claims to refute Part 2 of my Optics; and I foundself-contradictions in each of those. I can’t form opinions
about his achievements except on the basis of what I hold in
my hand.
I beg Fermat’s defenders to believe that if they are rightthat there’s some personal animosity between him and me,
it comes entirely from his direction. I aim never to have agrudge against those who try to prove that I’m wrong aboutsomething, ·especially· in a battle where it is no disgrace tolose. When I see that Fermat has friends who work hard to
defend him, I conclude that they are drawn to him by someattractive characteristics that he has. . . .
[The letter ends with a further page expressing
Descartes’s extreme irritation with people who criticise hiswork without understanding it—some of this being aimed

against Fermat, 1.iii.38:

off by the Roman Catholic Church’s condemnation of work by Galileo.]

Finally, you ask about the publication of my Physics andMetaphysics. Briefly: No-one wants that more than I do,
but only under certain conditions without which I would
be foolish to want it. I will say also that I am deeply calm
about the risk of their containing anything against the faith.
Indeed, I’m vain enough to think that my principles can
support the faith more strongly than any human arguments
up to now. Especially the doctrine of transubstantiation [seeGlossary], which the Calvinists say can’t be explained by theordinary philosophy, is very easily explained by mine. But it
doesn’t look as though the conditions that could oblige me
to do so [he means: ‘the conditions that would make it safe for me topublish that work’] will be fulfilled any time soon; so I settle fordoing whatever I regard as my duty, leaving everything elseto the Providence who rules the world; knowing that it is he
who gave me the small beginnings of which you have seen
the samples, I expect him to give me the grace to complete mywork if that would be useful for his glory; and if it wouldn’t be,
I give up all desire to do it. [In the preceding sentence, providence
could be referred to as ‘it’ rather than ‘he’ etc.; French doesn’t distinguishthese.] I assure you that the sweetest return I have had frommy publications is the self-approval that your letter causes
in me. It is especially precious and welcome to me because
it comes from a person of your worth who is also a priest
and is at the very place [the College of La Flèche] where I had thegood fortune to receive my entire education in my youth, and64

Correspondence

René Descartes

in Fermat’s direction. He comments on the difficulty thatfriends sometimes have in acting as impartial judges, andremarks that Hardy and Mydorge are the only two people inParis whom he would trust to judge properly in the present
matter]L

1638–1640

not sure enough about the details to be able to ‘thank’ himin the way he deserves. As for the discourses written by
him and his like: please treat them as nothing, and tell theirauthors that I am doing the same. Above all,please don’t agree to send any writing by anyone
unless its author says in writing that he agrees tomy publishing it along with my reply. . . .
After seeing Fermat’s last letter, which he says he doesn’twant published, I very explicitly asked you not to send me
any more letters of that sort. Of course if a Jesuit or a priestof the Oratory, or anyone else who was incontestably honest
and level-headed, wanted to send me something, a little morecaution would be needed. I’ll be entirely at the disposal ofsuch a person, but not of those spiteful characters whose
aim is anything but the truth. . . .
[Two pages of epistolatory politics. •Two of Fermat’s
friends have persuaded Mersenne to slow down the exchanges between him and Descartes, for the worst of reasons, Descartes says. •Descartes is enclosing his reply tocomments by Mydorge, and lists five other things that hewants passed along with it. Then:]
As for my arguments for the existence of God, in due
course they will be at least as highly regarded as any otherpart of the book. Vatier makes it clear that he appreciates
this point. His last letter shows that he fully approves ofeverything I have written; and that’s as much as I could wish
from anyone. So that what you had been told about him is
improbable.
I’m surprised that you should tell me that my reputation
is at stake in my reply to Fermat [letter to Mersenne of i.38
page 57]. In that reply, I assure you, there’s not a single wordthat I would wish to change, except the slips I pointed out
to you and others that you can recognise by the erasures.
You should speak of noting down the points in my book that

[1.iii.38: Descartes writes to Mydorge, enclosing copies of the whole

Fermat-Descartes interchange and asking for his judgment on it. The
dossier includes Descartes’s four-page account of the main things wrong
in Fermat’s latest letter (to Mersenne), a letter ‘to which I haven’t been
willing to reply, for a reason that you’ll see’. He asks that all this bepassed on to Hardy, with a request for his judgment.]

to Mersenne, 1.iii.1638:
I owe a reply to three of your letters, namely those of 8.i and
8 and 12.ii. The last of these I received only today, and thefirst only a week ago. I’ll reply in due order to the particularpoints that call for an answer; but on a more general note,
I must first •thank you for alerting me to many things thatit’s important for me to know, and •assure you that so far
from being upset by the bad things that are said aboutme, I rejoice in them—the more extravagant and outrageous
they are, the more I count them in my favour. . . . These
spiteful people wouldn’t go to such lengths to speak ill of me
if there weren’t others speaking well of me. Besides, truth
sometimes needs to be contradicted in order to be better
recognised. But I can only laugh at those who speak without
reason or justification.
As for de Beaugrand, I’m surprised that you condescend
to speak of him, after the way he treated you. [Descartes
believed, mistakenly, that Beaugrand had retained the MS of Descartes’
Discourse on the Method, passed on the MS of the Optics to Fermat
without permission, held back the licence to publish, etc.] I’d be glad if

you would give me an account of that affair once more;. . . .I’m
65

Correspondence

René Descartes

you regard as falsified by experience; I’m surprised, because
I venture to assure you that there aren’t any. I made all
the observations for myself, including the one you mention
concerning hot water freezing faster than cold water. What
I said in the book concerned water that has been heatedover a fire for a long time. Take some water that has beenthus treated, and some water that hasn’t, with both now at
the same temperature: the former will freeze faster thanthe latter. [Descartes follows this with some impatient
remarks about people who don’t—perhaps can’t—perform
experiments accurately.]
I am sorry to hear that Galileo’s eyesight has failed. I
am sure he would not think ill of my Optics, though I don’t
mention him by name in it.
[Dismissive remarks about ‘your analysts’, none of whom
understand Descartes’s Geometry; he names several. He
asks Mersenne to send along their criticisms, subject to the(indented) condition stated earlier in this letter. ‘And tellthem that after I have seen their writings my level of esteem
for them will be as high as they deserve.’ Further remarks
about individual persons and about the designing of lenses.]

1638–1640(i) If I had said without qualification that we should holdto opinions that we have once decided to follow, even though
they are doubtful, I would indeed have been as much toblame as if I had said that we should be opinionated andstubborn. . . . But that’s not what I said. I said that we mustbe •decisive in action even when •undecided in judgement,
and that we should follow the most doubtful opinions just as
steadily as if they were quite certain [Discourse, start of Part 3].
By this I meant that once we have settled on the opinion that
P which we judge doubtful—i.e. once we have decided that P
has no rivals that we judge to be better or more certain—weshould act on P with as much constancy as if we knewthat it was the best, which indeed it is when so considered.
There is no danger that this constancy in action will lead
us further and further into error or vice, since there can beerror only in the intellect, which—I am supposing—remains
free throughout and regards as doubtful what is doubtful.
Moreover, I apply this rule mainly to actions in life which
admit of no delay, and I use it only provisionally, intending
to change my opinions as soon as I can find better ones, andto lose no opportunity to look for them. Finally, I was obliged
to speak of this firmness and resolution in action for two
reasons: •for the sake of ease of conscience, and •to head off
a criticism that might be ·wrongly· aimed at my saying that inorder to avoid rashness we should must once in our lifetime
put aside all the opinions we have hitherto believed—namely
the criticism that such a universal doubt could give rise to
great indecision and moral chaos. Altogether it seems to me
that I couldn’t have been more careful about this, placing
the virtue of decisiveness between its two contrary vices,
indecisiveness ·in action· and obstinacy ·in belief·.

L[9.iii.38: Descartes writes to Huygens, a variety of remarks about
people—Campanella, Fromondus, Plempius, Fermat, and others.]

to Reneri for Pollot, iv or v 1638:
Your friend need not have been so ceremonious. People ofsuch worth and intelligence need no formal introduction,and I will always count it a favour when they do me thehonour of consulting me about my writings. Please tell yourfriend not to hesitate to do so. This time, however, since hewanted it so, I will ask you to pass on my replies to him. [This(2) [This refers to Discourse on the Method, early in Part 3 (‘The third
maxim. . . ’).] It doesn’t seem to me a fiction, but a truth that
nobody should deny, that there’s nothing entirely within

responds to the letter of ii.38, starting on page 57.]

66

Correspondence

René Descartes

our power except our thoughts; at least if you take theword ‘thought’ as I do, to cover all the doings of the soul,so that not only meditations and acts of the will, but theactivities of seeing and hearing and deciding on one ·bodily·
movement rather than another, so far as they depend on thesoul, are all ‘thoughts’. In philosophical language, nothingis strictly attributable to a man except what is covered by
the word ‘thought’; purely bodily events are said to happen
in a man rather than to be performed by him. Notice too theword ‘entirely’ and what came after it: ‘After we have done
our best in dealing with matters external to us, whatever
we fail to achieve is absolutely impossible so far as we areconcerned.’ This shows that I didn’t mean that externalthings are not at all in our power, but that they are in our
power only in so far as they can be affected by our thoughts;
they aren’t absolutely or entirely in our power because otherpowers, outside us, can frustrate our designs. To makemyself clearer I even put side by side the two expressions‘absolutely’ and ‘so far as we are concerned’, which a critic,
if he did not understand the sense of the passage, mightcomplain contradicted each other. Nothing exterior, then,is in our power except in so far as it is at the command of
our soul, and nothing is absolutely in our power except ourthoughts. But though this is very true, and no-one could
find it hard to accept when he thinks of it explicitly, yet I
did say that it’s a belief one has to grow accustomed to, andthat long practice and repeated meditation are necessary
to do so. This is because our desires and our passions areconstantly telling us the opposite. As children we found thatby crying or commanding we could make our nurses obey us
and get what we wanted; and this happened so often that wehave gradually convinced ourselves that the world was madeonly for us, and that everything is our due. Those who areborn to greatness and fortune are the more likely to deceive

1638–1640

themselves in this way; they too are commonly seen to be
the most lacking in patience—·i.e. in willingness to put up
with it, to take it without whining·—when they have to bear
misfortune. It seems to me that there’s no better occupation
for a philosopher than to accustom himself to believe whattrue reason tells him, and to beware of the false opinions
that his natural appetites urge upon him.
(3) When someone says ‘I am breathing, therefore I exist’,
if he wants to prove he exists from the fact that his breathing can’t occur without his existence, he proves nothing.
His ‘proof’ requires him first to establish that he really isbreathing, which he can’t do without also proving that heexists. But if he aims to prove his existence from his feelingor belief that he is breathing, so that he judges that even
if the opinion were untrue he couldn’t have it if he didn’t
exist, then his proof is sound. For in such a case the thoughtof breathing is present to our mind before the thought of
our existing, and while we have that thought we can’t doubtthat we have it. [Discourse on the Method early in Part 4.] To say‘I am breathing, therefore I exist’, in this sense, is simply
to say ‘I am thinking, therefore I exist.’ You will find on
examination that all the other propositions from which we
can thus prove our existence reduce to the same one; sothat we can’t prove from them the existence of the body, i.e.
of a nature that occupies space, etc., but only that of thesoul, i.e. of a nature that thinks. Of course one may wonder
whether the nature that thinks may perhaps be the same as
the nature that occupies space, so that there is one nature
which is both intellectual and corporeal; but by the method
which I suggested, it is known only as intellectual.
(4) From the very fact that we conceive vividly [see Glossary]and clearly the natures of the body and the soul as different,
we know that in reality they are different, and consequently
that the soul can think without the body, even though, when67

Correspondence

René Descartes

they are joined, its operation can be disturbed by the baddisposition of the bodily organs.
(5) The Pyrrhonists [= ‘ancient extreme sceptics’] didn’t infer
any certain conclusion from their doubts, but that doesn’tmean that no-one can. I would try right now to show
how these doubts can be used to prove God’s existence,by clearing up the remaining difficulties in what I wrote, if itweren’t for the fact that someone has promised to send mesoon a summary of everything that can be doubted on this
topic, which may put me in a position to do it better. So I
must ask the person who wrote these queries to let me delay
my reply until I have received that summary.
(6) Most of the actions of animals resemble ours, andthroughout our lives this has given us many occasions tojudge that they act by an interior principle [see Glossary] likethe one within ourselves, i.e. by means of a soul that has
feelings and passions like ours. All of us are deeply imbued
with this opinion by nature. Whatever reasons there may
be for denying it, expressing this denial publicly involves
exposing oneself to the ridicule of children and fools. But
those who want to discover truth must above all distrust
opinions that were instilled in them as children. In order toknow what we ought to believe on this question, it seems
to me, we should think about what answer to the question
would be accepted by a possible man ·whom I’ll call Homme·:
Homme has been raised from infancy in a place where
he has never seen any animals except men. He loves
the study of mechanics, and has made or helped tomake various automata shaped like a man, a horse, adog, a bird, and so on, whichwalk and eat and breathe and (so far as possible) imitate all the other actions of the animals
they resemble, including the signs we use toexpress our passions, like crying when struck

1638–1640

and running away when subjected to a loudnoise.
Sometimes Homme can’t tell the difference between
real men and automata that only have the shape ofmen, and has learned by experience that there are
only the two ways of telling them apart (I explained
these late in Part 5 of my Discourse on the Method):(i) the automata never answer in word or sign, except
by chance, questions that are put to them; and (ii)although their movements are often more regular andcertain than those of the wisest men, in many things
that they would have to do to imitate us they fail moredisastrously than the greatest fools.
Ask yourself: what will Homme think when he sees theanimals that we have? I stipulate that he is filled with theknowledge of God, or at least has noticed how inferior thebest human workmanship is to the workmanship of naturein the composition of plants. Nature has packed plants
with countless invisibly tiny ducts through which certain
juices gradually rise to the ends of the branches, where
they intermingle and interact and dry out in such a way
as to form leaves and flowers and fruits. Homme notices
this, and so believes firmly that if God or nature were to
make automata to imitate our actions they would •imitate
them more perfectly and •be incomparably better constructedthan any that men could come up with. Now when Hommesees the animals we have, and notices in their actions thesame two things that make them unlike us and that he has
already noticed in his automata, what will he think? He
won’t conclude that there’s any real feeling or emotion inthem; rather, he’ll think they are automata which, being
made by nature, are incomparably better than any of his
own past productions. Then there’s one last question. As
between68Correspondence

René Descartes•the verdict he gives, with knowledge of the facts and

1638–1640

to anyone who knows •that fire has the power to move theparticles of terrestrial bodies that it approaches (we often see
this happening), and •that hence it must move the particles
of subtle matter even more easily because they are •smallerand •less closely joined together, these being the two qualities
that entitle one body to be called ‘more subtle’ than another.
(11) Of course I don’t claim to be certain that the particles
of water are shaped like certain animals, but only that they
are elongated, smooth and flexible. If some other shape
can be found that would explain all their properties just as
well, I’ll be happy to adopt that instead; but if no others
can be found, I don’t see what difficulty there could be inimagining them to have that ·eel-like· shape. They must havesome shape, and the one I suggested is particularly simple.
As for the constitution of air: perhaps some air-particles
might also have this shape, but they can’t all have it. Why
not? For many reasons: •air wouldn’t be as light as it is,because particles with that shape can fit closely together withlittle space around them, thus constituting a fairly bulky
and heavy body such as water; and •air would be much
more penetrating than it is, for we can see that it is hardly
more penetrating than water, and in many cases even lessso. . . .and so on.
(12) The point being made in this paragraph seems to me
to be on a par with this:
I say that the pain we feel when cut by a sword is not•in the sword in the way it is in our senseorgans, but is simply•caused by the shape of its edge or point, by the
hardness of the matter the blade is made of,and by the force with which it moves.
Then someone objects that •other bodies with thatsort of edge could also cause pain; •that bodies withdifferent shapes, especially those that are soft and not

unprejudiced by any false opinion, and•the judgment we made when we were children, andhave retained only through habit,
which is more credible? We base our judgement solely on
the resemblance between some exterior actions of animals
and our own; but this is in no way sufficient to show that
there’s any resemblance between the corresponding interior
actions.
(7) I tried to show that the soul is a substance reallydistinct from the body. This is sufficient, I believe, indiscussion with people who believe God to be creator ofall, to force the admission that our souls must be necessarily
created by him. [See note on page 58.] And those who acquire
certainty of God’s existence in the way I have shown cannot
fail to recognise him as universal creator.
(8) I didn’t say that light was extended ‘like a stick’ but‘like the actions or movements transmitted by a stick’. And
although the movement lasts through time, each of its parts
can be felt at one end of the stick at exactly the same timethat it is produced at the other end. And I didn’t say thatlight is like grape juice in a vat; I likened it to the action
whereby the parts of the juice at the top tend to move towards
the bottom: these parts tend to move towards the bottom in
a completely straight line, though they can’t move exactly in
a completely straight line, as I said early in Optics.
(9) Since I made a point of not explaining the foundationsof my physics [Discourse, a third of the way through Part 6], I saw
no need to explain subtle matter more clearly than I did.
(10) Even though water remains liquid simply because its
particles are kept moving by the subtle matter surrounding
them, this doesn’t prevent its becoming liquid when its
particles are set in motion by some other cause. This
paragraph ·in the Discourse · should present no difficulty69Correspondence

René Descartes

hard like a sword, can’t be felt; and lastly that •thepain is nothing in the sword except its external shape,and isn’t an internal quality and that •the force thatprevents the sheath from breaking when the sword isinside it consists simply in the action through which
it wounds, and in its shape.
This ·analogous case· makes it easy to see how I’ll respond tothe objection ·to what I said about the taste etc. of salt·:
namely that bodies whose particles have the same size,
shape, hardness, etc. as those of salt will have the same
taste as salt. They won’t be tasteless; for something’s being
tasteless consists not in its lacking a sensation of taste within
itself but in its lacking the power to cause such a sensation.
And liquids whose particles have some other shapes or sizes
etc. don’t taste like salt but may have a taste—a less strongor sharp one if their particles are softer, just as the painof a bruise is not the same as that of a cut; and we can’t
cause as much pain with a feather as we can with a sword.
Lastly, I don’t see why taste is regarded as more an intrinsic
quality in salt than pain is in a sword. And as for the powerof salt to keep meat from rotting, this is due not to •its
sharpness or •the shape of its particles but to •the hardness
and inflexibility of its particles, just as it’s the inflexibility ofthe sword that prevents its sheath from breaking.
[Descartes responds also to the objector’s items (13)–(15).
His treatment of (15) spelling, is memorable:] It’s up to theprinter to defend himself on this score; because my only
instruction to him was to follow customary usage. Just as
I didn’t make him take out the p from corps or the t from
esprits when he put them in, so I didn’t bother to get him
to add them when he left them out, for I did not notice any
instances where this could create ambiguity. Anyway, I’m
not aiming to revise French spelling, and I wouldn’t advise
anyone to try to do this in a book printed in Leiden! But I do

1638–1640

have a view about this: I think it would be much easier forforeigners to learn our language if the spelling followed thepronunciation exactly. . . .

to Huygens, 9.iii.1638:
Regarding the book by Campanella that you sent me: Fifteen
years ago I saw his book on The Meaning of Things alongwith some other treatises by him, perhaps including thepresent book. But I found so little substance in his writings
that I now can’t recall anything about them. All I cansay about them now is that •those who go wrong ·on theirown· through fondness for the most out-of-the-way paths
are worse—less excusable—than •those who go wrong incompany by following the well-trodden paths.
As for Fromondus, the little disagreement between him
and me wasn’t worth telling you about. . . . Our dispute
was conducted like a game of chess: we remained goodfriends once the match was over, and now we send eachother nothing but compliments. Plempius also produced
some objections against ·my account of· the movement of
the heart; but he did this in a friendly way, his aim being topromote discovery of the truth. I try to reply to each critic in
the style in which he writes to me. A Counsellor of Toulouse[Fermat] also raised some objections against my Optics andGeometry. Some friends of his in Paris wanted to act as his
seconds, but if I’m not mistaken neither he nor they could
get out of the duel without admitting that everything theysaid against me was logically defective. I didn’t venture tosend you any of these objections, because I didn’t think it
worth your while to read them; and copying them would havebeen a tiresome chore; and anyway they may be publishedquite soon. In fact I would like many of my critics to attackme in this way, and I won’t complain about the time it will
70

Correspondence

René Descartes

take me to answer them until I have enough to fill a complete
volume; for I’m sure this is a pretty good way of showing
whether what I wrote can be disproved.
I would have been particularly pleased if my opponents
had included some Jesuit priests; and letters from L’Isle,
La Flèche, and Louvain led me to expect that they would
be. I did get a letter recently from someone at La Flèche[Vatier], who writes in terms as glowing as one could wish for,and then goes on to complain not about •the explanations
I gave but about •the ones I didn’t give, pressing me topublish my Physics and Metaphysics. The Jesuits are inclose correspondence with each other; so the testimony ofjust one of them is enough to make me look for them all to
be on my side. But for all that, I can’t see any hope of giving
my World to the world in the near future. Without that, I
can’t complete the Mechanics that you wrote to me about,because that depends entirely on the other works, especiallyin connection with velocity. And we have to expound thelaws of nature and explain how they work in ordinary events
before we can adapt nature to operations that are quite outof the ordinary. I have nothing to say about Pollot’s request
to see the three sheets, except that you may do as you please.
It is more than courteous of you to leave me some rights insomething that belongs to you. . . .
[One version of this letter has a paragraph here in which
Descartes thanks Huygens for sending him a book, andcomments negatively on its content. Its author says that
Descartes’s philosophy follows that of Democritus, andDescartes says: ‘I don’t know whether that is right, because
I don’t trust the things we are told about that ancientphilosopher. He seems to have had a really good mind,
and not to have been capable of the irrational doctrines thatare attributed to him.’]
I understand that young Gillot is in The Hague. If I could

1638–1640

recommend anyone to you, it would be him, for he was myfirst (and practically my only) pupil, and the one with thebest head for mathematics.
L[iii.38: Plempius writes to Descartes, explaining why he is not satis-

fied by two of the points in Descartes’s answer (15.ii.1638) to his earlier
objections.]

L[iii.38: Ciermans writes to Descartes. He is a colleague of Plempius;and argues with Descartes through six pages, not about biological matters (as Plempius did) but about physics, especially light and colours.]

L[23.iii.38: Descartes replies to Plempius, seven pages entirely devoted

to theories and observations relating to the movements of the heart andof blood close to it.]L

[23.iii.38: Descartes replies to Ciermans, a dozen Latin pages mainly

on light and colours.]

L[31.iii.38: Descartes writes to Mersenne, 17 pages responding tovarious comments by others, reported in two letters of iii.38 of which
we now have neither. The main topics are as follows. •Beaugrand’s
claim that Descartes’s geometry is a mere copy of Viète’s. In rebutting
this, Descartes gives a memorable account of what he was up to in therelevant parts of the Geometry: ‘I was making the construction in theway architects make battlements, merely prescribing what must be done,
and leaving it to carpenters and masons to do it.’ •Someone’s ‘ignorantor malicious’ accusation that Descartes in his optics has borrowed from
Kepler. While rejecting this, Descartes says ‘I don’t deny that Kepler is
my chief master in optics’. •An experiment that Descartes himself has
performed with the eye of a just-killed cow, investigating the possibility ofseeing in the dark. •Problems in geometry and number-theory proposed
to Descartes by Fermat and his two supporters. •Scattered through theletter are messages to various of Descartes’s critics—Morin, Ferrier, Petit,
Desargues, Gibieuf—about what they should do and how Descartes will
respond.]L[iv.38: Roberval writes against Descartes, 11 pages of scathing criti-

cism of the Geometry and of Descartes’s replies to Fermat.]71

Correspondence

René Descartes

from Mersenne, 28.iv.38:

Fermat’s that has been sent to him. That material isn’t worth the time ittakes, Descartes says, but he’s afraid that his delay in responding to it[He starts by crediting Roberval with many fine discoveries
in geometry, and states one of them. Then:]
Please allow me to present two questions that Roberval
and I are currently disputing. What would the state of affairs
have been if God hadn’t created anything?
Roberval: There would have been the same real threedimensional space that there is now. The eternal
truth of geometry is based on this space, whether or
not God fills it with bodies.
Mersenne: There would have been no space. Otherwise space is a real being that doesn’t depend on
God.
[The second question is a complex one concerning the velocity, at various stages in its journey, of a bolt shot from across-bow. Then:]
We are also in difficulties over the question:•Why is a cannon-ball less damaging at 15 or 20 feetfrom the cannon than it is at 50 feet? and•When I throw a stone, why does it hurt you less if itmeets your body just after leaving my hand than it
would if you were a dozen strides away?
It seems that either (a) the effect of these missiles depends
less on their speed than on some other factor, or (b) eachmissile speeds up in the course of its journey. You don’t
accept (b), and neither do I. I’m sure that if you walk once
gently around your room, that will give you enough time toclear up this puzzle for us.
[Mersenne then produces a problem in geometry sent by
Fermat; some others of his own; and a geometrical question
which he is currently arguing about with Desargues.]L

1638–1640

may be harming his relations with Mersenne. ‘So I shall tell you all mythoughts about it, once for all so that I shan’t need to think about it any
more.’ Then ten pages of geometry.]

to Mersenne, 27.v.1638:[Replying to Mersenne’s 28.iv, Descartes starts with embarrassed gratitude for the trouble Mersenne has taken on hisbehalf (‘26 pages in your own hand’), remarks that Roberval’sreported discovery in geometry is no big thing, and says thatthe same is true of all he has so far seen of the work ofFermat and his defenders. He continues:] They have sentme a great Register of Fermat’s discoveries; but this, rather
than making me think better of him or them, has remindedme that pauperis est numerare pecus [Latin, from Ovid: ‘It’s thepoor man who counts his sheep’]. . . .
You ask whether there would be real space if God had
created nothing. This question may seem to be beyond thecapacity of the human mind, like infinity, so that it’s nota reasonable thing to argue about; but in fact I think thatit’s merely beyond the capacity of our imagination, like thequestions of the existence of God and of the human soul. I
believe that our intellect can reach the truth of the matter,
which is (in my opinion, anyway) that •there wouldn’t be any
space, and that •even the so-called ‘eternal truths’—such
as The whole is greater than its part wouldn’t be truths if
God hadn’t established them as such. I think I have alreadywritten to you about this. [See the letters to Mersenne of 15.iv.30,
6.v.30 and 27.v.30 on pages 15–17.][On the question about the cross-bow Descartes agrees
with Roberval except for one detail. Then:] I’m not yet certain
about the experiment to discover whether a cannon has less[3.v.1638: Descartes writes to Mersenne, responding to his request(in a 26.iii letter that we don’t have) that Descartes evaluate work of72

Correspondence

René Descartes

1638–1640•my assumption that it is an action or power ·to act·
which •gets from place to place following the samelaws as movement from place to place, and •affects

force close up than further away. I believe that the effectwill vary according to the kind of material the cannon-ballhits—not having the same effect on a metal breast-plate as
it has on a pine plank—although it goes faster when leaving
the cannon than afterwards. . . .
You ask [in a later letter which we don’t have] if I regard what
I have written about refraction as a demonstration. I think
Yes, at least to the extent that •demonstrations can begiven in this field without first bringing in metaphysics todemonstrate the principles of physics (which I hope to dosome day); and to the extent that it has ever been possible
to demonstrate results in mechanics, optics, astronomy,or anything else that isn’t pure geometry or arithmetic.
Asking me to give geometrical demonstrations on a topic thatinvolves physics is to ask for the impossible. And if nothingis to be called a ‘demonstration’ except geometers’ proofs,
then we’ll have to say that Archimedes never demonstrated
anything in mechanics, or Vitellio in optics, or Ptolemy inastronomy. Of course nobody ever says this. In such matters
we’re satisfied if •the authors’ assumptions don’t obviously
conflict with experience and •their discussion is coherent
and free from logical error, even if their assumptions aren’tstrictly true. I could demonstrate, for instance, that even
Archimedes’ definition of the centre of gravity is false, andthat there is no such centre; and others of his assumptions
are also not strictly true. The assumptions of Ptolemy andVitellio are even less certain, but that isn’t a sufficient reason
for rejecting the demonstrations they have based on them.
Now what I claim to have demonstrated about refraction does
not depend on•the truth about the nature of light, or on•whether light is propagated instantaneously,
but only on

distant places through an intermediary, namely an
extremely rarefied fluid that the pores of transparent
bodies contain.
Your difficulty about affecting something in an instant arises
from an ambiguity in ‘instant’. You seem to take it as denying•every kind of priority, as if the light of the sun could turnup heresans passer premièrement par tout
l’espace qui est entre lui et nous;
which literally means: without first passing through all theintermediate space;but Descartes probably meant: except in consequence of
passing through all the intermediate space;what comes next:

but ‘in an instant’ excludes only •temporal priority; it iscompatible with each near-to-earth part of a ray of lightdepending on all the further-from-earth parts, in the same
way as the end of a time-taking movement depends on all
its preceding parts. There are only two ways to refute what I
have written: (i) to prove by experiments or reasoning thatmy assumptions are false; or (ii) to show that what I havededuced from them doesn’t really follow. Fermat understood
this very well; he tried to refute what I wrote about refraction
by attempting to prove that (ii) it contained a logical error. As
for those who settle for saying that they don’t believe what I
have written because I deduce it from certain assumptions
that I haven’t proved—they don’t know what they are asking
or what they ought to ask. . . .
[Descartes is exasperated by the low quality of what Petit
wrote to him: a jumbled farrago of points about God andthe soul, not having understood a word of what Descartes
73

Correspondence

René Descartes

had written on this topic. Petit is better qualified to write on
optics, but there’s evidence—Descartes gives some—that heis incompetent or ignorant in that area also. Then:]
My opinion of Morin is quite different. I think I would be
indebted to him for his objections, as I would be to all who
make a point of telling me that their aim is to see that thetruth is discovered. I won’t be hostile to them if they treat
me as roughly as they can, and I’ll try to answer them all sothat none has cause to be offended.
[•A complicated paragraph about Descartes’s role in helping the advancement of a favourite ex-pupil named Gillot.
The complications come from the fact that the young man’sparents are staunch Protestants while his best professional
opportunities are in Roman Catholic countries or institutions.
•A message to his critics about what’s going on when they
attack him roughly and he replies roughly. •Thanks to
Mersenne for sending something by Gibieuf; and complex
remarks about which of his critics Descartes will arrange tohave published, and why. •Dismissive remarks about Fermat
and Roberval. •More about Gillot, his possible role as an
explainer of Descartes’s work to folk in Paris. •Appreciative
words about a musical theorist named Bannius. •A fast
rattle of remarks about various other people. Then:]
Between ourselves, the ·social· atmosphere of Paris is theworst for my plans, because of the countless distractions
that are inevitable there. For as long as I’m allowed to choose
my way of life I shall live in the country, in a state of peace inwhich I can’t be bothered by visits from neighbours. That’s
what I have now in this corner of Holland. That’s my only
reason for preferring this country to my own, and I am now
so used to it that I have no desire to change. . . .
L

1638–1640

on to Huygens, presumably because the Leiden Librarian, Heinsius, is afriend of the Huygens family. He warns Huygens that Heinsius has to behandled gently, and quotes a light-hearted letter from Balzac reportingthat something friendly that he wrote about Heinsius was ‘received asoutrageous’ because Heinsius can’t take a joke.]L[3.vi.38: Descartes writes to Mersenne, continuing the dispute withFermat and Roberval, and then ten pages answering mathematical questions that had been asked by a correspondent of Fermat’s named SainteCroix and then presumably passed on to Descartes by Mersenne. At
the end of this he says ‘Spare me!’ and begs Mersenne not to send him
any more questions taking up time that he can’t afford. He adds that
‘I have never claimed to know anything about numbers’, and that he has
forgotten much of what he used to know.]L[vi.38: Descartes writes to Hardy. He is glad that Hardy is on his sidein the tussle with Fermat over a certain mathematical rule that Fermat
has proposed. He then goes on to say that the rule can be corrected andgiven a proper foundation, and he spends three pages doing that.]L[29.vi.38: Descartes writes 22 pages to Mersenne, mostly picking

over matters arising from the letters of 27.v and 3.vi. Also: ‘You ask
me if foreigners have made better objections than the French, to which I
reply that Morin is the only French critic I have had.’ He doesn’t countPetit because his ‘objections’ were so bad and irrelevant.

He allegesthat Fermat and his friends have entered into a conspiracy to discredit
Descartes’s writing. And some harsh words about de Beaugrand, which
Descartes will try to tone down in (17) on page 82]

to Morin, 13.vii.1638:
The objections you have taken the trouble to send me areones I’d have been glad to get from anyone; but your rank
among the learned, and the reputation your writings haveearned you, make them more pleasing from you than fromanyone else. My best way to show you this is (I think) by
carefully answering you on every point.

[vi.38: Descartes writes to Huygens. Descartes’s friend Hardy has

asked him to help him get permission to borrow two books in Arabic thatare now owned by the Leiden Academy, and Descartes is passing this74

Correspondence

René Descartes

You begin with my assumptions. You say:‘The phenomena of the heavenly movements can be
deduced just as certainly from the assumption thatthe earth is stationary as from the assumption that it
moves.’
I agree readily. I hope I’ll get the same understanding ofwhat I wrote in the Optics about the nature of light, so thatthe force of the mathematical demonstrations I tried to set
out there won’t ·be thought to· depend on any opinion inphysics, as I said sufficiently clearly in my Optics. If there’s
some other way of imagining light that will explain all theproperties of it that we know from experience, it will be seenthat everything I have demonstrated about refraction, vision,and so on can be derived from that just as well as from theassumptions I made.
You say also:
There’s a vicious circle in proving effects from a cause,and then proving the cause by the same effects.
I agree: but I don’t agree that it is circular to explain effects
by a cause and then prove the cause by the effects—there’s
a big difference between proving and explaining—and I add
that the word ‘demonstrate’ can be used to signify either, if it
is used according to common usage and not in the technicalphilosophical sense. I should add also that there’s nothing
circular in proving a cause by several effects that are independently known, and then conversely proving certain othereffects from this cause. I have combined these two senses
in my Discourse on the Method: ‘As my last conclusions
are demonstrated by the first, which are their causes, so
conversely the first are by the last, which are their effects.’
But that doesn’t show me speaking ambiguously, because
I went straight on to explain what I meant, saying thatexperience renders most of these effects quite certain, thatin deducing causes from them I’m not so much proving them

1638–1640

as explaining them—indeed it’s the causes that are provedby the effects. And I put ‘not so much proving them’ rather
than ‘not proving them at all’ so as to make the point that if
there were any doubt about any of these effects it could also
be proved from this cause, provided the cause had alreadybeen proved from other effects. I don’t see what other terms
I could have used to explain myself better.
You say also that astronomers often make assumptions
that cause them to fall into grave errors; as when they makewrong assumptions about the parallax, or the obliquity of theecliptic, and so on. To this I reply that those aren’t the sort ofassumptions or hypotheses I was speaking of; I marked outthat sort clearly when I said that one could draw very trueand certain consequences from them even though they werefalse or uncertain. The parallax, the obliquity of the ecliptic,
and so on can’t be assumed as false or uncertain, but only
as true; whereas the equator, the zodiac, the epicycles andother such circles are commonly assumed as false, and the
movement of the earth as uncertain, and yet for all that, true
conclusions are deduced from them.
Finally, you say that nothing is easier than to fit a cause
to an effect. It is indeed easy in many cases to fit cause toeffect, one on one; but it’s often harder to fit a single cause to
many different effects if it isn’t the true cause that producesthem. There are often cases where one can prove what is thetrue cause of a number of effects simply by giving one fromwhich they can all clearly be deduced. I claim that all thecauses I of spoke belong to this class. If you(i) bear in mind that in the whole history of physics up
to now people have only tried to imagine causes to
explain the phenomena of nature, with virtually no
success, and(ii) compare •the assumptions others make of ‘real[= ‘thing-like] qualities’, ‘substantial forms’, ‘elements’75Correspondence

René Descartes

and the like with •my single assumption that all bodies
are composed of parts, this being something that isvisible to the naked eye in many cases and can beproved by countless reasons in others, and(iii) compare my deductions from my assumptions—about vision, salt, winds, clouds, snow, thunder,
the rainbow, and so on—with what the others havederived from their assumptions on the same topics,
this will be enough, I’m sure, to convince any unbiased person that the effects that I explain have no causes except theones I have deduced them from; although I don’t demonstrate
this now, saving it up to present in another place. [Descartes
builds into (ii) the confession that his explanations involvea further premise, namely ‘that the parts of certain kinds ofbodies are of one shape rather than another’, but this isn’tmuch of an addition because ‘it’s easy to demonstrate it toanyone who accepts that bodies are composed of parts’.]I’m sorry that your objections all concern light, because I
have decided not to state my views on that topic in my replies;
and I don’t want now to reverse that decision. So I shan’t beable to answer you as thoroughly as I would have liked. But
please believe me: I wasn’t trying to hide behind a barricadeof obscure expressions as a defence against a sneak attack,as you seem to have thought. If I have a certain skill inmathematical •demonstrations, as you do me the honourof saying I have, •they are more likely to have taught me
to discover the truth than to have taught me to disguise it.
I didn’t speak as openly about light as about the other topics
because was my decision not to include anything in these
Essays that I had already tried to explain in exact detail in
another treatise [namely The World ]. . . .
You have been told that I despise the scholastics: thismust have been dreamed up by people who don’t knowme or my mœurs: [see Glossary] or my temperament. In my

1638–1640

Essays I hardly ever used terms that are familiar only tothe learned, but this doesn’t mean that I disapprove of suchterms—merely that I wanted to use other terms so as tomake myself understandable. The bottom line is this: it’s
not for me to select the weapons I am to be attacked with;my task is only to try to defend myself. To do that, I’ll reply
now to each of your points separately. [The numbering of points
is Morin’s and then Descartes’s.][(1)–(3) sort out ‘misunderstandings’ of things Descartes
said about light and subtle matter, explaining seeminginconsistencies as signs of his alertness to what needs to besaid at each particular point in his exposition.](4) You raise two objections to a certain passage in the
Optics. The first is that ‘if light is only an action or inclination
to move, it is not in that case a movement’. Where did I say
it is a ‘movement’ without immediately adding ‘or an action’?
I don’t believe there is any such statement in my writings,
and especially not when I discussed the sort of light thatcan be seen in transparent bodies, which philosophers calllumen in Latin so as to distinguish it from the light thatcan be seen in luminous bodies, which they call lux. Now,
when I say in some places that light is a movement or an
action, and in another place that it is only an action, there’s
no contradiction in that. Also (and this is important) the
meaning of the word ‘action’ is general: it covers not only
•the power or inclination to move but also •the movement
itself; when we say ‘He is always in action’, we mean that
he is always moving. That’s how I am using the word in thecontext you cite; so there is no ambiguity there. . . .
The second objection you make here is that ‘if the action
belongs to subtle matter, it does not belong to luminous
bodies’. But this objection rests upon an ambiguity in theword ‘light’. I readily admit that the action of subtle matter,
which is lumen, is not an action of luminous bodies, which
76

Correspondence

René Descartes

is lux; but that isn’t an admission that what I said wasambiguous, for I was very careful to distinguish between
these two senses of ‘light’ throughout. [(5)–(6) concern purely
verbal points.](7) I’m surprised that you cite pages 4–5 of Optics in orderto prove that the movement of luminous bodies can’t get
as far as our eyes—that nothing material given off by thesebodies is transmitted. All I’m trying to do in those two pages
is to expound the analogy with a blind man, which I put
forward primarily to show how movement can be transmitted·from one place to another· without anything moving ·from
one to the other·. I don’t believe you would think that when
the blind man touches his dog with his stick, he can’t sense
its movements unless it—the dog itself—passes along the
stick to his hand! But to answer you in formal terms: when
you say that there can’t be movement without somethingthat moves, I make a distinction. The movement can’t indeed
occur without some body, but it can be transmitted from one
body to another, and thus pass from luminous bodies to oureyes through the medium of some third item, namely. . . .theair and other transparent bodies or. . . .a very subtle matterthat fills the pores of these bodies and extends without abreak from the stars to us. . . .
(8)–(9) You prove quite convincingly that the round particles of the subtle matter cannot exactly fill all the pores ofterrestrial bodies. I agree; but it doesn’t follow from this thatthe space •they don’t occupy is empty; for the pores could beoccupied by •something else, which I needn’t go into here.
[Nine pages, mostly on points concerning light and/or
subtle matter. Descartes deals with most of them by claimingthat he has been misquoted or misunderstood. Then:]At the end you ask: ‘Does your view imply that the force
with which a spark from a fire or a glow-worm at night mustpush the subtle matter towards our eyes, if we are to be able

1638–1640

to sense the light, can be impeded by the force of a windblowing hard in the opposite direction?’ [In his reply to thisDescartes travels through a detour to this conclusion:] In nocase can the motion of the wind impede the action of light,
except when its motion is so violent that it sets fire to theair; and then the light that is created can obliterate the lessintense light of a spark. . . .
But in my view, your main objection—which is perhaps
why you decided to keep it till the last—is this:‘If the pores of transparent bodies must be straight,
it seems that they can’t let the subtle matter passthrough them in every direction, because a solid
body can’t possibly contain straight pores in every
direction.’
I can clear up this difficulty by means of an analogy, if we
don’t take the word ‘straight’ in a stricter sense than I wasclearly intending to take it. What I said [he gives page-numbers]
is not that the pores must be perfectly straight, but that theymust be only as straight as is needed for the subtle matter toflow right through without meeting any obstacle. [Descartes
then gives an analogy which he thinks might help; but the
explanation of what he means by ‘straight’ is enough on its
own.]
The part of your letter that is hardest for me to reply tois its conclusion. I don’t claim to deserve the kind words
you have applied to me, but I am not up to rejecting them.
So I can only say that like you I deplore fortune’s mistakein not sufficiently recognising your merit. But as for me,
thanks be to God fortune hasn’t so far helped or hinderedme; and for the future I don’t even know whether I should•want fortune’s favours or •fear them. I regard it as dishonest
to borrow something from someone and not return it withinterest; so I would be deeply in the red if I felt that I was indebt to the public. As for the malignant people you speak of,77

Correspondence

René Descartes

I believe that other centuries have had at least as many ofthem as this one has; and I am positively pleased when they
attack my works, because I see them as like flies or birds
that always go for the best fruit. But I thank you for the good
fortune you have wished me, and for the trouble you havetaken to write to me.

1638–1640

lead in a vacuum, each having the same quantity of matter,they would stay equally balanced. They also have the notionof ‘relative weight’: when two bodies x and y contain the sameamount of matter, they say, x may seem to be heavier thany because in x the matter is more compressed—i.e. spreadover a smaller space—than it is in y.
Of these two views, (a) is the one most commonly heldin the Schools [see Glossary], (b) is most in favour with those
who think they know more than ordinary folk. On boththese views, it is obvious that a body’s absolute heaviness isalways intrinsic to it and thus always the same, not varying
according to the body’s distance from the centre of the earth.
There is also the view (c) that all heaviness is relative.
On this view the force or power that causes the bodies wecall ‘heavy’ to descend is not •in them but •in the centre of
the earth or •in the earth’s entire mass, which attracts them
towards the centre—as a magnet attracts iron, or in someother way. Since a magnet and every other natural agent
with a given sphere of action is more active at close range
than at long range, this view implies that a body’s weightincreases as it gets closer to the centre of the earth.
My own conception of the nature of heaviness is quite
different from each of those three. But I can’t explain it
without going into many other lines of theory that I don’tplan to discuss here. I can only report that it doesn’ttell me anything that bears on the Question, except that
the Question is a purely factual one, i.e. a question thathuman beings can’t definitively answer unless they can bringexperiments to bear upon it. [An extremely obscure andpuzzling sentence expressing pessimism about our being
able to get reliable results by experiments. Then:]An experiment that we can make requires a tall •towerwith a very deep •shaft at the foot of it, a weight—•·a brick,for example·—attached to a long cord, and •a balance:

to Mersenne, 13.vii.1638:
An investigation of the question:
Does a body weigh more when close to the centreof the earth than it does when far from it?
[In what follows, ‘the Question’ will always refer to the above question.]

We must distinguish here between two sorts of heaviness:•true or absolute heaviness,•apparent or relative heaviness.
When we say that a staff weighs much more when we hold it
at one end than when we grasp it in the middle, we’re talking
about apparent or relative heaviness; we’re saying that in
the one case it seems heavier, or rather is heavier from ourpoint of view, not that it is intrinsically heavier. But before
discussing this relative heaviness, we must define what wemean by ‘absolute heaviness’. Most people take it to be a
power or quality inherent in every body that we call ‘heavy’,making it tend towards the centre of the earth. Some thinkthat (a) this quality depends upon the body’s form, so that a
portion of matter that is heavy when it has the form of water
loses the quality of heaviness when it takes on the form of
air ·by turning into steam·. Others hold that (b) heaviness
depends only on the matter, so that every body is heavy,
because every body is composed of matter. According tothese (b)-theorists, how heavy (absolutely speaking) a given
body is depends on how much matter it is composed of; they
imagine that if we could weigh a mass of air and a mass of78

Correspondence

René Descartes

At the top of the tower we •weigh the brick and thecord with the whole thing in one pan of the balance,
and then we •tie one end of the cord to the pan and let
the brick hang down to the bottom of the shaft, and•compare the results of the two weighings.
The difference between them should tell us whether the
brick’s weight is significantly different when closer to thecentre of the earth than it is when further away. But thedepth of the shaft plus the height of the tower will be tiny
compared to the radius of the earth, and that’s one reason(there are others) why

1638–1640

as cranes, swans, etc. fly much more easily when high inthe air than when nearer the ground. Might it be due tothe force of the wind? No, because the same thing occurs
when there is no wind. So we have reason to think thatthese birds are made lighter by their greater distance fromthe ground. Paper kites flown by children, and all the snowthat the clouds hold, provide further evidence for this view.
There is also the observation [mentioned on page 29] that you
told me you have made yourself, and that other writers havedescribed, namely that cannon balls that are shot straight
up don’t fall down again. If that really is what happens, we
can only suppose that the force that shoots the ball upwards
sends it so far from the centre of the earth that it loses its
heaviness. So much for the physics of the Question.
I turn now to the mathematical arguments, which canapply only to relative heaviness. To conduct such arguments,
we need a settled value for the absolute heaviness ·of whatever body is involved in the calculation·. We can’t get thisby •discovering what its absolute weight is; to have a settled
value we’ll have to •suppose it by making an assumption. So
let us pick some distance D and stipulate that the absoluteweight of a body x is the force with which x tends in a straightline towards the centre of the earth when it is•at distance D from the earth and•in our ordinary atmosphere and•neither pushed nor supported by any other body and•not yet moving.
I say ‘in our ordinary atmosphere’ because if x is in a thinner(or thicker) air than our own it will certainly be a little heavier(or less heavy). And I say ‘neither pushed nor supported byany other body’ and ‘not yet moving’, and ‘at distance D fromthe earth’, because all these factors can affect the force withwhich x tends to move downwards. The ‘distance D’ is to beunderstood as a constant—the same in all our calculations.

how the sentence ends: cette expérience ne pourra servir, si

la différence qui est entre un même poids, posé à diverses
hauteurs, n’est fort notable.
which means: this experiment won’t be instructive unlessthe difference between the results of the two weighings isvery noticeable.
what Descartes should have said: the absence of any percep-

tible difference between the two weighings won’t entitle us toinfer that NO is the answer to the Question.
[The point is that Descartes hasn’t said anything to justify the claim thata small difference between the two weighings wouldn’t be relevant to theQuestion.]

Another observation provides, I think, very powerfulevidence that bodies far from the centre of the earth don’t
weigh as much as those closer to it. The planets that have nolight of their own—e.g. the Moon, Venus, Mercury etc.—areprobably bodies composed of the same sort of matter as
the earth; and the heavens are liquid, as most present-day
astronomers hold. So one might think that the planets
should be heavy and fall towards the earth, but becauseof their enormous distance ·from the earth· they have lost
all tendency to do this. Also, we see that large birds such
79

Correspondence

René Descartes

Moreover, we shall suppose that each particle of a given
heavy body always has a given force or tendency to descend,whether it is far from the centre of the earth or close to it, andno matter how it is situated. As I have already remarked, thisassumption is perhaps not true; but we ought to make it inorder to simplify our calculations. It’s like how astronomers
assume that the average motions of the stars are equal, inorder to make it easier to calculate the true motions, which
are unequal.
Given this assumption of the equality of •absolute heaviness, we can demonstrate that the •relative heaviness of all
hard bodies when they are in the open air and not supported
by anything is somewhat less when they are close to thecentre of the earth than when they are far from it (although
this doesn’t hold for liquid bodies). If two perfectly equalbodies are placed in the opposite pans of perfectly accurate
scales whose arm isn’t horizontal, the body closer to thecentre of the earth will weigh more; and the differencein weights will be exactly proportional to the difference inproximities to the centre of the earth. . . .
The proof of this point depends on just one principle,
which is the general foundation of the whole of statics,
namely that it takes neither more nor less force to raisea heavy body to a certain height than it takes to raise aless heavy body to a greater height, or a heavier body to alesser height; and the difference in height is in both cases
proportional to the difference in weight. [See page 50 above.]
For example, a force that can raise a weight of 100 pounds
to a height of two feet can raise a weight of 200 pounds toa height of one foot, or a weight of 50 pounds to a height offour feet, and so on. . . .
You’ll have no difficulty in accepting this if you consider
that an effect must always be proportional to the action thatis needed to bring it about, and hence that

1638–1640

if to raise a body x to a height of one foot we need aforce that can raise a 100lb body to a height of twofeet, this shows that x weighs 200lb.
For raising a 100lb pound body one foot twice over is thesame as raising a 200lb body one foot, and as raising a 100lb
pound body two feet. It obviously follows from this that •therelative heaviness of each body—which is the same as theforce needed to support it and prevent it from descending—is
to be measured by the beginning of the motion that thepower supporting it must produce if it is to raise it. . . . So
the ratio between the straight line that this force describes
and the line that indicates by how much the body moves
closer to the centre of the earth is equal to the ratio between
the absolute weight and the relative weight. [Descartes gives
three examples: the pulley, the inclined plane and the lever.
His accounts of the pulley and the lever are the same as
those beginning on pages 50 and 51. The account of theinclined plane is different, but it extremely hard to follow
and requires a diagram that can’t be provided here; so we’ll
have to let ourselves off from reading it, and rejoin Descartes
as he sums up:]
Now these three examples are, I think, sufficient to convince us of the truth of the principle that I put forward, and toshow that all the points usually discussed in statics dependon it. For the wedge and the screw are simply inclined planes;
the wheels used to construct various sorts of machines are
simply multiple levers; and the balance is nothing but a leversupported at its centre. Thus, all that remains for me to
explain here is how the two conclusions I put forward can bederived from this principle. [Descartes now offers an obscure‘demonstration explaining the sense in which a body can besaid to weigh less when nearer the centre of the earth thanwhen further away’. After giving it, he remarks that it holds
only for solid bodies, and that for liquids a different account80Correspondence

René Descartes

has to be given. He gives it, and then revisits ‘the sense inwhich. . . ’ etc. giving a different ‘demonstration’ of it.]

1638–1640

have reason to fear going to the stake!) Also, their professed
opposition to ·all· vices requires them to blame the desire
to speak badly of someone, and Petit is at least as muchpossessed by •that desire as the most pious of them couldbe •by God’s love. As for me, people of judgment who know
me won’t expect me to shrink from responding to him if I
think that will do any good; but I can tell you that I wouldfind it as shameful •to write against a man of that sort as •tointerrupt my walk by chasing away a puppy barking at mein the street.
(2) This doesn’t stop me from wanting to clarify thearguments I gave for the existence of God, but I’ll do this in
Latin.
(3) Most of the objections ·to the Discourse, Optics, etc.·that I have been sent, and that I plan to publish when enoughof them have come in, are also in Latin. So I would be glad
if those who intend to send me objections in future would
write them in Latin.
(4) I’m inclined to think that I shall be sent some from theJesuits of la Flèche. If so, they will prefer to write them in
Latin rather than in French; so I would be glad if you would
let them know ·of my preference for Latin for this purpose·;but do this as a casual passing remark, not an outrightdeclaration, because perhaps they aren’t planning to sendme any.
(5) I would also like to know how they deal with myMeteorology in their philosophy—do they try to refute it orjust keep silent about it? They certainly don’t follow it; thepublic theses they are putting forward this season make thatclear.
(6) I’m obliged to Desargues for taking the trouble toshow that he’s sorry I don’t plan to continue my studies
in geometry. But all I have decided to give up is abstract
geometry, i.e. research into problems that are useful merely

to Mersenne, 27.vii.1638:[On 3.vi.38 [page 74] Descartes had written to Mersenne
answering a series of mathematical questions that Mersenne
had received from Sainte-Croix and passed on to Descartes.
This letter begins with Descartes expressing appreciation forSainte-Croix’s reception of these answers, and says that helearned from it. Then eight pages of mathematics, continuing
the discussion with Sainte-Croix, followed by:]I am glad to hear that Sainte-Croix has promised torespond to the comments on his offerings that I sent toMydorge on 1.iii.38 [page 65], because I expect that when he
examines my reasons he’ll come to recognise that what henow calls ‘subterfuges’ are really very certain truths that
I have used in responding to his sophisms. And if many
people don’t understand my demonstration, it shouldn’t
be inferred that the demonstration isn’t evident, but onlythat the material is difficult. The works of Apollonius andArchimedes contain many demonstrations that are very
evident, although many honest and otherwise-able people
couldn’t understand them. . . .
This letter up to here has been addressed ·through you·to others. Now I turn to various items in your own letters.
(1) Petit has told you that the general assembly of the
Capuchins unanimously expressed admiration for what hehad written against me. I could only laugh at this; it’s not
likely that the devotion of these good friars makes them sosimple-minded that they •can’t see the irrelevancies and false
judgments that appear on every line of what Petit wrote and•approve of his impieties. (Some of these are so gross that if
he were in a country where the Inquisition is active he would
81

Correspondence

René Descartes

as mental exercises. That will give me more time to workon another sort of geometry, where the problems concern
the explanation of natural phenomena. If Desargues thinks
about what I wrote about salt, snow, rainbows, etc., he’ll seethat my entire physics is nothing but geometry.
(7) He wants to know my views about the minute particles
of bodies? Then let me tell you that I imagine them simply
as being like •the stones that make up a wall or •the planksa ship is made of. That is, it’s much easier to separate themfrom one another than to smash any one of them or put it
together again or change its shape. We can of course do allof these things, provided we have the appropriate tools.
[(8)–(16) deal with a variety of personal and secretarial
matters that needn’t concern us. Then:](17) What I wrote about de Beaugrand [in a letter of 20.vi.38,
see page 74] is nowhere near to being worth publishing; but I
gather from what you tell me that there is a wish for it to be
published. This doesn’t matter much to me as long as myname is kept out of it and two changes are made:•Replace the words: ‘He shows here that his ignorance
is matched by his shameless impudence’ by something
like ‘He shows here that one shouldn’t put much trustin what he writes’.
•Replace the words ‘This book on is so irrelevant, so
ridiculous, and so contemptible. . . ’ by ‘This book is
so contemptible. . . ’.
It’s not that the first versions don’t fit him; but it isn’t fitting
for me to write them. . . .
PS: I am enclosing with this my letter to Fermat, unsealed.
Please seal it before you send it on to him.
L

1638–1640

explained it more clearly. He mentions some problems he still has withthis work, but optimistically expects that Fermat will solve them. We
don’t have the letter of Fermat’s to which this replies.]

L[30.vii.38: Huygens writes to Descartes, replying to his letter of vi.38(see page 74), saying that he wrote to Heinsius a month ago and that,after a misunderstanding was cleared up, Heinsius agreed to lend Hardy
the books. Remarks about certain personal relationships. Report on
the claim of the popular philosopher van der Straten to be able to bringdiamonds or gold into existence on a person’s palm, and to perform otherwonders. Huygens asks for Descartes’s opinion about whether anythingin nature would permit these things to happen.]L[1.viii.38: Mersenne writes to Descartes expressing pleasure over thequality of what Descartes wrote to him and to Morin in his letter of
13.vii.1638—see page 74—and reporting that Descartes’s reply to Morin
had done a lot to raise him in Morin’s estimation because it showed that
Descartes doesn’t ignorantly despise Aristotle’s philosophy. ‘There arepeople who are misled by the compactness and clarity of your style—adopted so as to be understood by ordinary folk—into thinking that you
don’t understand scholastic philosophy; and I tell them that you know it
as well as the most self-important of the masters who teach it.’]L[12.viii.38: Morin writes to Descartes in flattering terms (‘the mostsubtle and productive mind of this century’), and explains why he
had written to Descartes about light (see Descartes’s letter of 13.vii.38,
page 76): his own research requires him to understand light, and that
he was especially anxious to get Descartes’s views about light •because
they are so splendidly new and •because the concept of ‘subtle matter’that they involves is basic and central in Descartes’s physics. Then 14
pages of responses to that letter of Descartes’s—objections, requests forclarification, etc.]

L[27.vii.38: Descartes writes to Fermat expressing (in florid terms)[23.viii.38: Descartes writes to Mersenne, 12 pages of mathematics,

great pleasure in receiving from Fermat a letter offering friendship.

and then: ‘As for Fermat, I hardly know how to reply to him. After the

Descartes also refers to a geometrical procedure of Fermat’s that he hadcompliments we have paid to one another I would be sorry to displeasepreviously dismissed, describing it as ‘very good’ now that Fermat hashim. But it seems to me that the passion with which he •continues to82

Correspondence

René Descartes

finding scraps of knowledge hidden in the corners of libraries;
and if scrap-finding is all he is good for, he won’t be able tochoose and order what he finds. It’s true that the author says
he has already started such a book, and I can well believethat he can make a better job of this than anyone else; but
the specimens he presents here don’t inspire confidence. The
content of the aphorisms on pages 31ff is so •general thathe seems to have a long road to travel to reach •particulartruths—which are all we need for practical purposes.
Besides this, I find two things in his programme that
I can’t entirely approve. (i) He seems to want to combine
religion and revealed truths too closely with the sciencesthat are acquired by natural reasoning. (ii) He imagines auniversal science that could be learned by young scholars
before they reach the age of 24. He seems not to notice that
there’s a great difference between the two sorts of truths:•Knowledge of revealed truths depends only on grace,
which God denies to no-one, though it isn’t efficacious
for everyone; so that the stupidest and simplest folk
can acquire it as well as the most sophisticated.
•To have any chance of doing something extraordinary
in the human sciences you have to have an extraordinary mind.
It’s true that we’re obliged to make sure that our reasonings
don’t lead us to any conclusions that contradict what God
wants us to believe; but I think that we’ll be misusing
Scripture—using it for a purpose that God didn’t assign
to it—if we try to derive from the Bible knowledge of truths
that •belong only to human sciences and •don’t contributeto our salvation. . . .

praise his own method and •to maintain that I have misunderstood it
and have gone wrong in what I wrote ·in my letter of i.38—see page 57·—
obliges me to set down here some truths that seem to me to go against
him.’

Then eight pages of that, followed by ten pages of comments

on various scientific matters involving various opponents/supporters ofDescartes’s work.]

L

1638–1640[viii.38: Descartes writes to Plempius, politely continuing the dis-cussion of the movement of blood through the heart, and declaring
Plempius’s objections to Descartes’s view about this as much better than
Petit’s.]to Hogelande, viii.1638:
I have read carefully the book you kindly sent me, and I
thank you for it. The author [Jan Amos Comenius] is clearly
an intelligent and learned man, of great integrity and public
spirit. All his criticisms of the accepted sciences and teaching
methods are only too true, and his complaints are only too
justified.
His plan of collecting into a single book everything thatis useful in every other book would be very good if it werepracticable; but I don’t think it is. It is often very hard tojudge accurately what others have written, and to extractfrom it the better parts without also taking some bad parts
too. Moreover, the particular truths scattered through thebooks are so detached—so independent of each other—that
it would take more talent and energy •to assemble theminto a well-proportioned unity (which is what your authoraims at) than •to create such a unified body ·of doctrine·out of one’s own discoveries. I don’t mean that we should
neglect other people’s discoveries when useful ones turn up;
but I don’t think that collecting them should be anyone’s
main occupation. If someone is capable of finding thefoundations of the sciences, he oughtn’t to waste his lifeL[viii.38: Descartes writes to Huygens, replying to his ’s letter of

30.vii.38 (see page 82). Huygens’s original letters are elegantly written,
with literary allusions, jokes, puns etc., and Descartes here acknowledges that fact. The favour that Huygens does for Descartes in writing83

Correspondence

René Descartes

pushing it or pulling it with equal force. What each man feelsin the staff—namely its lack of movement in the different
cases—can be called the various actions that are impressedon it by the other man’s exertions. ·They really are different
actions·, because when one man is pulling the staff thisdoesn’t cause the other to feel the same action as when he ispushing it, etc.
[Descartes now says a bit about points (5)–(6), and then:](7) One body can indeed push another body in a straight line
without itself moving in a straight line. Consider for example
a stone being swung around in a sling: the stone pushes thepouch in the middle of the sling, and thus pulls the attachedcord in straight lines that tend in all directions from thecentre of its motion towards the circumference. Now, to set
out more fully what I was then trying to say, I now say thatmy view is this:Sunlight is composed solely of a highly fluid sort ofmatter which continually revolves around its centre
at a very great speed, thus pressing on all sides thematter that makes up the heavens, which is simply thesubtle matter that the sky is made of, which stretches
uninterrupted from the stars to our eyes. It’s throughthe medium of this matter that we come to feel thepressure of the sun that is called ‘light’.
I think this should remove most of the difficulties that you
presented. Of course you could immediately raise many otherdifficulties about this point, but I would have just as many
answers to them—indeed I have them already prepared!
—and we wouldn’t be finished with this affair unless I set out
my entire physics.
(8)–(10) To prove that subtle matter exists, I need only to
get you to consider that •there are pores in many perceivablebodies (visible to the naked eye in wood, leather, paper, etc.);that •these pores don’t have to be empty just because they

to him is supported (Descartes says) by ‘all the Muses of France’, but hecan’t in return to invoke the Muses of Leiden, and will express himself
plainly. The letter deals with •some personal relationships, •Descartes’s
belief that some people who pride themselves on being expert geometricians are trying to suppress Descartes’s work because they see it as athreat to their supremacy, and •the supposed feats by van der Straten
which he says are, though ‘rare’, not physically impossible.]L[12.ix.38: Descartes writes to Mersenne responding to a letter of

1.ix.38 (which we don’t have) reporting that objections had been raised
against ‘the principle I assumed in my treatment of the question ofwhether the earth moves’.

1638–1640

He devotes eight pages to answering theobjections.]

to Morin, 12.ix.1638:[This responds to Morin’s letter of 12.viii.38, which was an answer toDescartes’s letter of 13.vii.38. The numbering of items up to (10) follows
the numbering in both of those earlier letters.]

Given the fairness of your motives and the breadth of yourcourtesy, I think I am obliged to do my best to answer
thoroughly all the further points that you put to me.
You begin with my reply (4). I wasn’t denying that theword ‘action’ should be taken to mean ‘movement’; but theword also has a more general sense, including the sense of‘inclination to move’. Suppose two blind men are holding awooden staff and•they push it with equal force against each other, sothat it doesn’t move at all; and then•each pulls it with equal force towards himself, andagain the stick doesn’t move.
In each case there’s a force in one direction and anotherforce in the opposite direction, the forces being so exactly
equal that the staff doesn’t move. The very fact that it doesn’t
move enables each blind man to feel that the other man is84Correspondence

René Descartes

are too small to let air in; and that •therefore they must be
full of a matter that is more rarefied [see Glossary] than thematter composing the bodies I’m talking about. The various
movements of this subtle matter are shown well enoughby the movements of the bodies through whose pores it
passes. . . .
You say that
•if light is nothing but the action of the sun, then there
is no light in the sun’s nature; and that•light is a more actual and more absolute being thanmovement is; and that
•only God acts by his essence,and so on. You’re making difficulties in words where there
are none in reality. Any more than there would be a problem
than if I said that•a clock shows the time only by the movement of its
hands, and that•its quality of showing the time is no more an actualor absolute being than its movement is, and that
•this movement belongs to it by its nature and essence,
because it would stop being a clock if it didn’t have it.
I can hear you saying ‘But the clock’s form is artificial,whereas the sun’s form is natural and substantial’; but
I reply that this distinction concerns only the •cause of these
forms, and not their •nature. And if ·you avoid going thatway by saying that· the sun’s substantial form is different
from the qualities to be found in its matter, then this is aphilosophical entity that’s a stranger to me.
·You cast doubt on the usefulness of analogies, comparisons·. Well, it’s true that the comparisons scholastics
customarily use to explain•intellectual matters in terms of physical ones,•substances in terms of accidents [see Glossary], or•one quality in terms of a quality of a different kind

1638–1640

are not very instructive. But my only comparisons are of
movements with other movements, or shapes with othershapes; i.e. I compare things that are too small to beperceived by the senses with other things that can be so
perceived, differing from them simply as a large circle differsfrom a small one. I maintain that analogies of this sort
are the best means available to the human mind for laying
bare the truth in problems of physics. When someone says
something about nature that can’t be explained by any suchanalogy, I take that as a demonstration that what he says
is false. [What comes next is addressed to Morin’s challenge: Can
Descartes explain how a tiny spark, seen through a telescope 50 miles
away, can set in motion all the subtle matter between the spark and thetelescope?] As for the analogy of a U-shaped tube that I usedin my reply, I maintain that it shows that a small force canmove a great quantity of highly fluid matter. . . . To get reallyclear about this, imagine a tube encircling the earth, with nopart of it higher than any other except for a bit at each end
that sticks up enough to hold a tiny quantity of water. If wepour one drop of water into one of those two openings, thiswill set in motion all the water in the tube, even if the water
is otherwise no more inclined to move in one direction thanin the other—and the quantity of water is no less than thequantity of subtle matter that a spark sets in motion. . . .
[Then Descartes deals with several points concerningtransparency, defending himself against criticisms on pointsof detail. Finally:] At the end of your letter you remark thatwhen you see dust in the air dancing about in a sunbeam
you understand what I take the subtle matter to be. Thisshows that your thoughts on this matter are very different
from mine. The smallest particles of dust are much larger
than the particles of pure air, and the smallest particles
of air are much larger than the particles that I ascribe to
subtle matter, which I conceive of as a continuous liquid85

Correspondence

René Descartes

occupying all the space not taken up by other bodies, andnot as something composed of disconnected parts such as
the particles that make up dust.
L

1638–1640

origin—an excellent device for removing softness and other
weaknesses that the students may have acquired throughbeing habitually pampered in the parental home. . . . If yourson does come to these parts, I shall serve him in any way I
can. In Leiden I have lodgings in a house that would provide
him with good board; but I think that his studies would go
better at Utrecht; for the university there was founded only
four or five years ago, and hence hasn’t had time to go bad;and there’s a professor there named le Roy [Regius], who is agood friend of mine and, in my view, better than anyone atLeiden.

[ix.38: Descartes writes to Ferrier with some news about develop-

ments in the project of making hyperbolic lenses.]to Debeaune, 12.ix.1638:
I am much obliged for your kind remembrance of me, andI am honoured by your wish to have my opinion about theeducation of your son. I would encourage you to send him
to this country [the Netherlands] if I thought that your planfor his education could actually be carried out here; butphilosophy is badly taught here. All that the professors
do is to lecture for an hour a day for about half the year;
and they don’t dictate any written material, or complete thecourse within a set time. So those who have any desire
to learn have to get private instruction from a tutor, as dolaw-students in France. I don’t hold that all that is taught
in philosophy is gospel truth; but it is the key to the othersciences, so I think it’s worthwhile to take the complete
course in philosophy as it is given in the Jesuit schools
before trying to rise above mere book-learning and become
genuinely knowledgeable. And to give my own teachers theirdue, I must say that nowhere on earth is philosophy taught
better than at La Flèche. Moreover, to leave home for the
first time and suddenly find oneself in a country with adifferent language, religion and way of life is an enormouschange. The atmosphere of La Flèche, however, is very close
to your own; and since young people go there from all overFrance, their inter-relations create a mixture of different
temperaments that has almost the same educational effectas travel. Lastly, the Jesuits treat each other as equals, thehigh-born being treated much the same as those of humbler

to Mersenne, 11.x.1638:
I’ll begin this letter with my comments on Galileo’s book
·Discourses and Mathematical Demonstrations Concerning
Two New Sciences·. Generally speaking, he philosophises
much more ably than most—he •does his best to abandon
the errors of the Schools [see Glossary] and •tries to bringmathematics to bear on problems in physics. I am absolutely
with him on that, because I think that that’s the only way
to discover the truth. But he goes wrong in continuallydigressing, and in not pausing to explain matters fully. Thisshows that he hasn’t been orderly in his investigations, andhas merely tried to explain some particular effects without
digging down to the primary causes in nature; so that he isbuilding without having any foundation to build on. . . .
Page 11. Here he introduces the topics he intends todiscuss, namely: Why are large machines weaker than small
ones, given that they have exactly the same structure andare made of the same material? Why is a child less seriously
injured by a fall than an adult is? Why is a cat less seriously
injured by a fall than a horse is? I don’t think there is any
difficulty about this, any reason to construct a ‘new science’!
86Correspondence

René Descartes

It’s obvious that if a large machine’s resistance to being
broken up is exactly proportional to that of a small machineof the same shape, they can’t be made of the same material;
the larger must be made of material that is harder and lesseasily destroyed, in proportion as its size and weight arelarger. There’s as much difference between•a large machine and a small one made of the samematerial
as between•two large machines of the same size, one made from amuch lighter and harder material than the other.
Page 17. He is right when he says that the threads of astring stay together because they press against each other;but he doesn’t say why this pressure causes them to stay
together, namely that minute inequalities in the shape ofthe strands prevent each strand from sliding between thestrands pressing against it. . . .
Page 20. He presents two explanations for the fact thatthe parts of a continuous body hold together:•the abhorrence of a vacuum, and•a sort of glue or cement that holds them together,which he explains later on in terms of a vacuum. I thinkthat both of these are quite false. What he ascribes to a
vacuum should be ascribed only to the weight of the air. If it
were abhorrence of a vacuum that prevented two bodies fromseparating, no force would be capable of separating them.
And the method he gives to distinguish between the effectsof these two ·supposed· causes is worthless. . . .
All that he says about the infinite is wrong: he admits
that the human mind, being finite, can’t comprehend theinfinite, yet he goes ahead and discusses it as if he didcomprehend it.
Page 47. He says that when hard bodies liquefy they are
divided into an infinity of points; but he gives no evidence

1638–1640

for this fiction, which is easy to disprove. . . .
Page 43. His experiment to discover whether light istransmitted instantaneously is useless; for eclipses of themoon have an exact bearing on the calculation in question,and thus are clearly superior to any observations we could
make on earth. . . .
Page 153. He assumes that the speed at which a weightdescends always increases uniformly. I used to believe that
too, but I now think I have demonstrative proof that it’s
not so. . . .
Page 217. He adds another false assumption, namely:
bodies thrown up in the air travel at a uniform speed
horizontally, but as they fall their speed increases at a ratethat is proportional to twice the distance covered. It’s asimple matter to infer from this that bodies thrown up in the
air move along a parabolic path; but since his assumptions
are false, his conclusion may also be far from the truth.
[Descartes firmly criticises some things Galileo says about
the geometry of the trajectory of a cannon-ball in flight; andthen:] I’ll say nothing about the geometrical demonstrations
that most of the book is full of: I couldn’t summon thepatience to read them, and I am prepared to believe they
are all correct. But it did occur to me as I looked at his
propositions that you don’t need to be a great geometrician
to discover them. And I noticed that he doesn’t always takethe shortest route to his conclusion, which is a blemish in
his work.
I would be glad if this letter were seen by you alone. You
asked for my views, and I’m so greatly indebted to you that
I don’t think I should deny you anything within my power.
Otherwise I wouldn’t have spent time raking over someone
else’s mistakes, for that goes completely against my grain.
Also, if I had been writing for other eyes than yours, I would
have given reasons for my assertions more thoroughly than87

Correspondence

René Descartes

I have here, so that those who don’t know me as well as you
do couldn’t imagine that I had arrived at my views without
good reason.
I’ll turn now to the separate points that you have raised inyour letters; I have been slow to answer them because lately
I have had trouble staying awake ·in the evenings·. First,
concerning Galileo: I have never met him and have hadno communication with him, so I couldn’t have borrowed
anything from him. Anyway, I see nothing in his books
that stirs my envy, and hardly anything I would wish toacknowledge as my own. The best part is what he says aboutmusic; but those who know me would think it more likelythat he got it from me than that I got it from him; for I wrote
practically the same thing 19 years ago, when I hadn’t yet
visited Italy. What I wrote then I gave to Beeckman, who, as
you know, made a great thing of it, and wrote about it invarious places as if it were his own. . . .
[Then 12 more pages on lenses, oil, Fermat, Roberval, Petit, de Beaugrand, the introduction to Descartes’s Geometry,
Boulliau, places where there are echoes, and Galileo.]Lof the letter of 15.xi.38, with the main emphasis being on
mathematics. He corrects an error in something he wrote,saying ‘I must have been falling asleep when I wrote that’and quoting the Latin poet Horace, quadrate bonus dormitat
Homerus = ‘Even Homer sleeps sometimes’. Then:]
At last you understand ‘force’ in the way I do when I say
that it takes as much force to raise a 100-pound weight one
foot as to raise a 50-pound weight two feet—meaning thatit takes as much action or effort. I can believe that I hadn’t
explained this well on previous occasions, given that you
didn’t understand it. I was so far from thinking of ‘force’as the power that we call a man’s ‘force’ when we say ‘This
man has more force than that one’ etc. that it didn’t enter
my head that anyone would take it in that sense. And when
we say that one effect requires less force than another, thisdoesn’t mean that less power is needed, for there would be
no harm in having more power; it means merely that lessaction is required. In that paper, I was. . . .thinking only of
the action that we call the ‘force’ that can raise a weight,
whether the raising is done by a man, a spring, some otherweight, or the like. Now the only way to determine a priori[see Glossary] how much effect can be achieved by a given
action (i.e. how heavy a weight, and of what sort, can beraised by means of such-and-such a machine) is to measure
how much action causes this effect (i.e. the force required toraise such a weight). . . .
As for what Galileo writes about the balance and the lever,he expounds well •what the facts are but doesn’t explain•why those are the facts, as I do by my principles. And as
for those who say that I should have explained machines
in terms of velocity (as Galileo does) rather than of space,between ourselves I regard them as fantasists who have nounderstanding of the subject. Obviously it takes more force
to raise a body quickly than to raise it slowly, but it’s a mere

[11.x.38: Descartes writes to Fermat a lavish statement about Fer-mat’s excellence as a mathematician, with brief indications of disagreements on some points. There’s a passing reference to Roberval, ‘who iscertainly another of the leading geometers of our century’.]L

1638–1640[Morin writes to Descartes ten pages of continuing resistance on thematters they have been arguing about.]

to Mersenne, 15.xi.1638:[This 29-page letter responds to four from Mersenne thathave come in since Descartes’s letter ‘five weeks ago’. Thatletter isn’t mentioned in any of the four, making Descartes
worry that the package with his letter may have been lost in
the mail. The first dozen pages deal with many of the topics
88Correspondence

René Descartes

fantasy to say that the force has to be doubled if the speed
is to be doubled; it’s easy to show that that’s not so.
[A page of remarks regarding geometry, Fermat (respectful), Roberval (cool), and de Beaugrand (scathing). Then:]
How are we to make sound judgements about what
notions can be taken as principles? The only way I know is
to prepare our mind by •getting rid of all the opinions that
we are preoccupied by, and •rejecting as doubtful everythingthat could be wrong. It is a common notion [see Glossary]that if a thinking being doesn’t depend on anything else, thenit is God. Why? Because if something’s existence is due
to itself, we can’t •doubt that it will have given itself as
many perfections as it could recognise, or •believe that we
recognise any perfections that it couldn’t recognise. But thestatement that a purely material being doesn’t depend onanything else doesn’t imply that it is God.
I looked for the letter in which you quote the passage from
St Augustine, but I still can’t find it; nor have I managed toobtain the works of that Saint, so that I could look up what
you told me about. But thank you.
[Then three pages on a variety of topics and people:
whether Fermat was right in saying that Galileo had misunderstood a passage in Aristotle; Campanella’s new book(what Descartes has seen of it doesn’t make him want to see
the rest); tiresome behavior towards Mersenne by Descartes’s
brother; no response to be sent to Morin because he evidently
doesn’t want one (‘His views seem to be even further frommine than they were are the start of our correspondence’);postal arrangements for Mersenne’s letters to Descartes,
some very slow, others fast; a recent bit of geometry by
Debeaune—not in fact correct, but better than anyone else’s
treatment of the same topic.][Mersenne’s second letter is in three parts. The firstdescribes several experiments; Descartes’s says that •the

1638–1640

experiments on ‘the tube’ would have been better done by amethod he has described early in the present letter [omitted
from this version], and that •the exploration of differences in
the forces needed to break a cylindrical object across its
width and along its length was a waste of time—there are no
general truths to be discovered here. Then:]In the second part of your second letter you make remarks
about Galileo. I accept that what prevents the separationof contiguous terrestrial bodies is the weight of the cylinderof air resting on them. . . . But I don’t accept that the forceof the continuity of bodies comes from that source, because
this force consists simply in the connection or union between
their parts. [Here ‘connection’ translates liaison, which can also mean
‘cement’. It is used in that sense on page 87 where Descartes refers to
the theory that cohering bodies are held together by ‘a certain glue orcement’ (colle ou liaison), a theory that he declares to be false. If liaisonhas the same sense in both passages, they conflict. If instead we takethis latest use of the word to mean the abstract ‘connection’, that avoids

Why
did I say that if something occurred because of abhorrence ofa vacuum, no force could prevent it? Because the existenceof empty space is ·not merely something that nature ‘abhors’,
but is absolutely logically· impossible, just as it’s impossiblethat there should be highlands without lowlands.
I imagine the particles of subtle matter to be as hardand solid as bodies of their size can be; but since they can’t
affect our senses, and the names of qualities are relative to
our senses, such names can’t properly be applied to them.
Similarly, we don’t say that powder is hard and heavy, butrather that it is soft and light compared with pebbles; yeteach of its particles is of the same nature as a tiny pebble.
I don’t agree that rotten wood or a candle can be motionless when it is giving off light; it couldn’t give off light if its
tiny particles—or rather the particles of the subtle matterthe conflict but makes the present passage vague and puzzling.]

89

Correspondence

René DescartesLin its pores—didn’t move extraordinarily forcefully. I gave adetailed account of the cause of this movement and of thewhole nature of fire in my World, I didn’t want to discuss it
in my Essays; I couldn’t have made it intelligible in a few
words. I agree. . . .that •there can’t be rarefaction in one
place unless there is condensation somewhere else, and that•when a body expands in a furnace it’s easy to find somethingthat can undergo a compensating condensation, namely thesurrounding air, which can easily be compressed. . . .
[Descartes now continues discussions about the velocity
with which water falls through air, and other related topics.
One small episode in this passage is notable: ‘. . . a body thatmoves in a vacuum, i.e. in a space containing nothing but
matter that doesn’t speed it up or slow it down’.]Your third letter has to do with the Optics. I’m grateful foryour correcting the errors in it; I’ll be glad if you will kindly
mark the corrections in your own copy, so that you can send
it to me if there is a second printing. In choice of language
and spelling I merely want to follow ordinary usage; but it’s
so long since I was last in France that there are many things
I don’t know. . . .
[The rest of the letter concerns: •subtle matter and thepores it lurks in; •Debeaune’s geometrical work and the fools
who don’t understand it; •warnings against believing what‘charlatans’ say they have achieved in lens-making; •thewrongness of Fermat’s criticisms of the Optics, and thanks
to Mersenne for having challenged them; •some geometry
and physics in reply to questions Mersenne has asked;•rejection of the charge (reported by Mersenne) that the
Optics is ‘borrowed’ from Roger Bacon. Descartes concludes
the paragraph on Fermat by saying that his view of him is
improved by the fact that ‘he spoke only according to his
belief’.]1638–1640[xi–xii.38: Descartes writes to some artisan (or perhaps to Ferrier) re-garding details in the making of lenses. The tone is extremely respectful.]L[xii.38: Descartes writes to Huygens, a charmingly affectionate letter

just to make contact, with the excuse of Descartes’s having found on his
shelves a book belonging to Huygens.]

to Mersenne, xii.1638:[Descartes expresses several dissatisfactions with Fermat,
summing up thus:] I have seen many of his writings, inwhich I have found two or three good things mixed in withmany bad ones. Between ourselves, I think of them in theway Virgil thought of Ennius, when he extracted ·little bitsof· gold from his works under the title The dung of Ennius[in the background of that slur is a Latin idiom, aurum e stercore = ‘gold
from dung’]. But this is between ourselves, because I still want

to be his ‘Yours faithfully’ if he wants that.
[Descartes next •comments on someone’s objections to
the Optics, •thanks Mersenne for his labours to get copies
of Descartes’s work into the hands of various Italian cardinals, •asks for news of Gassendi, especially his opinionof Descartes’s work, and •answers the question ‘What’s so
special about the octave?’ Then:]
The reason why water stays in a watering-can ·that ispunctured at the bottom and sealed everywhere else· is not•abhorrence of a vacuum (for as you rightly say, subtle
matter might easily enter the can in place of the air) but •theweight of the air. For if water flowed out of the can and thespace vacated was taken up only by subtle matter, it would
have to raise the entire body of air right up to its highest
level.
As for air that is forced into a balloon by a pump: it
doesn’t become hard, though it makes the balloon hard.
What happens must be that the air-particles—which (unlike
90Correspondence

René Descartes

any subtle matter that may be there) are trapped in the balloon because they are too big to pass through its pores—are
pushed against one another and thus pressed out of shape,and in tending to regain their original shapes they act liketiny springs pushing outward against the sides of the balloon,
thus making it hard. That is what hardness is, namely adisposition to resist when pushed, whatever the cause is of
this disposition. . . .
I don’t recognise any inertia or natural sluggishness inbodies, any more than Mydorge does; and I believe that whena man walks he makes the entire mass of the earth moveever so slightly, because he is putting his weight now on
one spot, now on another. Yet I agree with Debeaune thatwhen the largest bodies are pushed by a single force (e.g. thelargest ships pushed by a single wind), they always move
more slowly than smaller bodies do. This might be enoughto confirm his arguments, without having recourse to this‘natural inertia’ that can’t possibly be proved. [Very approving
remarks about Debeaune; discussion of burning mirrors (seeGlossary); explanation of how highly polished bodies canbe different colours, namely that they are never so highly
polished that there aren’t differences of surface-texture andthus different colours.]L

1638–1640

lost the hot-headed aggressiveness that once attracted me tothe army, and these days my only ‘profession’ is Cowardice.
Moreover, I have acquired some little knowledge of medicine,and I feel very well and look after myself with as much careas a rich man with gout. So I almost think that I am now
further from death than I ever was in my youth. And if God
doesn’t grant me the knowledge to avoid the discomforts ofold age, I hope he will at least let me live long enough tohave free time in which to endure them. [The thought seems tobe: ‘Let me live long enough to finish my work; then I’ll be free to focus

Yet everything depends on God’s
providence, to which—joking aside—I submit myself withas much open acceptance as Father Joseph will have done
·when he died three weeks ago·. My ethical code tells me tolove life without fearing death.
(2) I’m extremely grateful for your care in correcting theprinters’ errors in my Essays, but I’m a bit afraid that it
won’t be useful: given how few copies have been sold (thepublisher tells me), there’s not much chance that he will
have to bring out a second edition. . . .
[(3) concerns practical problems in weighing air. Then:](4) If you conceive of God removing all the air in a room
without replacing it by any other body, then you had better beconceiving of the walls touching each other—otherwise you’ll
be thinking a contradiction. Just as we couldn’t imagine
God flattening all the mountains in the world while leaving
all the valleys, so we can’t think of him as removing every
kind of body while leaving space behind. Our idea of •body,or of matter in general, is contained in our idea of •space, i.e.
of something with length and breadth and depth, just as theidea of a mountain is contained in the idea of a valley.
(5) When I conceive of a body x moving in a totally
non-resistant medium, I’m supposing that all the parts ofthe surrounding liquid body are disposed to move at exactly
on the challenges of old age.’][9.i.39: Descartes writes to Frénicle, a warmly respectful discussionof some problems in number-theory.]

to Mersenne, 9.i.1639:(1) You tell me in your last letter that you and some othergood people are concerned about me when two weeks passwithout your getting a letter from me; on reading that, I
would have to be very weary of life if I neglected to lookafter myself! But by the grace of God I haven’t had any reallyserious illness during the past 30 years. Over the years I have91Correspondence

René Descartes

the same speed as x does, both in •making way for it and•moving in after it. That is why every kind of liquid allows
some movement or other. But to imagine matter that resisted
none of the various movements of different bodies, you would
have to suppose that God or an angel was moving its parts at
various speeds to correspond with the speeds of movementsin the body they surround.
I haven’t yet told you what I think prevents there beinga vacuum between the parts of the subtle matter. I couldn’t
explain it without bringing in another subtle matter, and I
wanted to save that for my World. But I’m too much in yourdebt to keep secrets from you, so I’ll tell you.

1638–1640

derstand this. (a) The smaller a body is, the less force isrequired to change its shape. If you have two balls of leadof different sizes you’ll need less force to flatten the smaller
than to flatten the larger; and if they collide, the shape of thesmaller one will change more than that of the other. (b) Whenseveral bodies are shaken up together, the smaller ones will
receive more of this motion, i.e. will move more quickly, thanthe larger ones. Both of these doctrines need the rider ‘otherthings being equal’. From this it follows demonstratively thatsince there are moving bodies in the universe, and since
there is no vacuum, there must be a type of matter whoseparts are so small and so fast-moving that the force of theircollision with other bodies is sufficient to change their shapeand mould them to fit the places they occupy. But I havealready said too much on a topic on which I didn’t intend tosay anything.
(6) Every experiment would be useful for something, if one
were engaged in studying the whole of nature. But I don’tknow of any that strike me as less useful than the study ofwhat forces are needed to break different cylinders. . . .
(7) I don’t think of the movements in subtle matter differently from how I think of the movements in all visible bodies.
The water of a river moves faster at some places than atothers, and sometimes flows straight and sometimes in a
curve, even though it is pushed along by the same force andmoves with the same flow; and the same is true of subtle
matter.
As for heat, it could be caused by the agitation of theparticles of this subtle matter, though strictly speaking it
consists only in the motion of terrestrial particles, because
they are what have the most force to move the particles ofother bodies and thus set them on fire. The more terrestrial
particles a body has, the hotter it can be—compare ironwith wood. A body’s terrestrial particles can be in rapid[Descartes is going to distinguish three kinds of matter, differing only
in how finely they are divided: (i) what he quite often calls ‘terrestrial(terrestre) matter’; all he means by ‘terrestrial’ is ‘not very finely divided’;
the other kinds of matter are also ‘terrestrial’ in the ordinary sense of theword; (ii) subtle matter such as has been mentioned often in previous
letters and in Descartes’s published works up to now; (iii) the ‘othersubtle matter’, even more finely divided, which he is now announcing toMersenne; Descartes has no name for it, but in the present version it will
be called ‘supersubtle matter’.]I have proof that in addition to the ·ordinary· matter thatmakes up terrestrial bodies there are two other kinds:•One is very subtle and has parts that are round oralmost round, like grains of sand; this fills the poresof terrestrial bodies and is the material of which all
the heavens [see Glossary] are made.
•The other—·supersubtle matter’·—is incomparably
more subtle still, and its parts are so small and sofast-moving that they have no fixed shape but at each
moment easily take the shape required to fill up all thelittle interstices that aren’t occupied by other bodies.
There are two things you have to know if you are to un92

Correspondence

René Descartes

motion, making it very hot, while the subtle matter in its
pores isn’t pushed about in the way needed for it to give
us any sensation of light. That’s why iron can be very hot
without being red-hot.
[Descartes’s next claim is that any portion of matter canbe •terrestrial at one time, •ordinarily subtle at another,and •supersubtle at yet another. Then in (8) he offers andexplanation of why iron is strengthened by being thrown into
cold water when it is red hot.](9) Why does a candle flame viewed at a distance in thedark appear much larger than it is? I can think of two
answers. (i) Since we can’t see its true distance, we imagine
it to be as far away as the stars—since the image of thecandle at the back of the eye is much larger than the imageof the star, we judge that the flame itself is larger. (ii) We see
not only the light coming directly from the candle but also
light coming from the dense air or other neighbouring bodies
that are lit up by it. It’s easy to distinguish these two sorts oflight at close range, but at a distance we ascribe them bothto the candle; so the flame seems bigger than it is. [Note that

1638–1640

For the rest of this winter I’ll be engaged in a study thatneeds me to be free of distractions. So I humbly ask you to
let me off writing anything between now and Easter (·25.iv·)
unless of course something urgent comes up. But go on
forwarding to me any letters that come to you addressed tome, and your own letters are always welcome. . . .

to Mersenne, 9.ii.1639:
Since you want me to respond to your letter of 28.i, I’m also
going to re-read the ones before it, so as not to leave anything
without a response. [Then seven pages dealing with•lenses: some made in Naples recently may owe something to Descartes’s work on this years ago;•crystals: Mersenne has sent some; Descartes is puzzled by how their-cross-section differs from that ofmost hexagonal crystals and from the shape of thecells of honey-combs;•fish: what makes it possible for them to swim andsteer themselves;•Fermat: Descartes doesn’t want to revisit a certain
work of his, because what it does is done better in his(Descartes’s) Geometry;•behaviour of Descartes’s family: deplorable;•Galileo: the physics of things’ sinking in water;•Descartes’s spelling mistakes;•subtle matter: adding something to the theory ofthem;•particles of ordinary matter: adding some things to
the theory of them;•Petit: his objections to Descartes on refraction are
merely comic;•centres of gravity;•supposed medical remedies: Descartes is scepticalin each case a fact about how something seems or appears is explainedin terms of things that we think or believe.]. . . .

[(10) concerns burning mirrors. In (11) Descartes saysthat he’s sorry to have led Mersenne to run certain experiments concerning the physics of fluids: he no longer
thinks that the outcomes would matter much, and in anycase] I find that it’s almost impossible to reason well on thebasis of experiments that I didn’t run myself, because every
experimenter has his own individual slant on what he isdoing. [(12) An experiment that Mersenne asked Descartes
to perform is judged by Descartes as not worth the trouble.
(13) Comments on some work by Desargues: Descartes saysthat it hasn’t been explained fully enough for him to have
any judgment on it.]93Correspondence

René Descartes

about some (which he names), but thinks that inhealthy people a superficial wound can usually be
cured by keeping it clean and bandaged;•Roberval: Descartes doesn’t want to see any more of
his work, and asks Mersenne to discourage othersfrom submitting their work to him;
•Desargues: his work on conic sections is an exception
because Descartes has obligations to him;
•Debeaune: his notes on Descartes’s Geometry are
another exception;•Gaudais: Descartes can’t make sense of what he has
written about trumpet-sounds.
Then:]
You tell me that an Italian medical man has written
against Harvey’s The Motion of the Heart, and that this makes
you sorry that I have committed myself to writing on this
topic. Frankly, I can’t feel grateful for your concern: you
must think very ill of me if, simply from being told thatsomeone has written something that you imagine to be
critical of me, you jump to the conclusion that I have gonewrong somewhere, without having seen his argument or evenknowing whether he is competent. (·I say ‘that you imagine
to be critical of me’ because its being against Harvey doesn’tmean that it is against me·. Those who take a superficial
view of things hold that what I wrote is the same as Harvey’s
view, simply because ·like him· I believe in the circulationof the blood; but my explanation of the movement of theheart is flatly contrary to his.) I can see from this andmany other such things that good arguments have very little
power to convince people of the truth. This almost persuades
me to give up writing altogether and to pursue my studies
exclusively for my own benefit. Still, I’m prepared to admitthat if what I have written on this topic or on refraction—or
on anything else that I have given more than three lines to in

1638–1640

my published writings—turns out to be false, then the restof my philosophy is worthless. I swear to you that it doesn’t
matter to me what people think of my work, especially now
when all they have are samples of it that nothing could bebuilt on. If I had given the whole thing ·to the world·, I am
sure I would have regretted it.
[The last six pages of this letter contain 16 numbered
items, all concerned with mathematics and/or physics
and/or practitioners of these sciences. Two of them arerequests to Mersenne not to send Descartes any more material from or relating to Fermat.]L[20.ii.39: Descartes writes to Debeaune, expressing great pleasure in

Debeaune’s notes on the Geometry. To show that he is capable of doingso, he points out some things in the Geometry that he (Descartes) thinks
are defective. Lenses, briefly. Five pages on Debeaune’s work on thegeometry of curves (‘better than Archimedes’).]to Mersenne, 20.ii.1639:[This letter starts with remarks about •Debeaune, •a geometer who says that he and Descartes studied Viète together inParis (Descartes doesn’t know this man and doesn’t think heopened Viète’s book in France), and •a question about how
many eggs would break if 50,000 of them were packed into abox. Then:]
The number and the orderly arrangement of the nerves, veins,bones and other parts of an animal don’t show that nature
is insufficient to form them, given that •in everything thisnature acts in accordance with the precise laws of mechanics,
and that •these laws were imposed on it by God. Indeed I
have taken into consideration not only what Vesalius andothers write about anatomy but also many things at a levelof detail that they don’t go into—things I have observed
while dissecting various animals. I have spent much time94

Correspondence

René Descartes

1638–1640

on dissection during the last eleven years, and hardly any
medical researcher, I believe, has looked at animals as closely
as I have. But I haven’t found anything whose formation
seems inexplicable by natural causes; I’m talking about thekind of explanation that I gave in Meteorology for the originof a grain of salt or a snowflake. In my World I started withthe fully formed body of an animal, and merely showed its
functions; but if I re-wrote that work I would undertake toinclude also the causes of the animal’s formation and birth.
But with all that knowledge I still don’t know enough to beable to heal a mere fever. I claim to know ·enough· about the
animal in general, which isn’t subject to fevers, but I don’tknow ·enough· about the man in particular, who is.

join the company; but his health is bad, and you have now
allowed me to visit you on my own. I hope to avail myself of
this permission during this last week of our vacation, and if
it wouldn’t be a burden to you I’ll spend two or three days
in your vicinity, so as to be able to consult you regarding
various plans that I have.

[The contrast here is not between ‘animal’ and ‘man’ but between ‘general’

Then five more pages of geometry.]L[30.iv.39: Descartes writes to Mersenne, acknowledging receipt ofseveral books and some letters. Remarks and advice relating to using
snow and salt to freeze water, why sunlight doesn’t reach the bottom of
the sea; also two pages of geometry in which Descartes mocks Mersenne
for repeatedly making the same mistake. Then this about Petit: ‘You
are doing too much honour to Petit by writing against him; we ought tolet such puppies snap at our heels without paying them any attention.’and ‘particular’. For Descartes, a man is an animal. He is contrasting•knowing a lot about animals in general with •knowing enough about

to Debeaune, 30.iv.1639:

any individual animal—e.g. an individual man (or horse)—to understandwhat is going on when he (or it) falls ill.]

The time I have spent studying your work on curves has
been well spent: I have learned a lot. . . . Thank you for yourcorrected measure of refractions; the previous one was sonear to right that no-one but you would have seen anything
wrong with it. As for the writing of Petit which ·you tellme· you have seen: I have less admiration for that—so little,
indeed, that ·if he wanted something to boast about· he could
boast of being the only person, out of all of those who havesent me things or written to me, that I haven’t responded to!
I would like to be able to meet your request concerning
your mechanics; but although my entire physics is nothing
but mechanics, I have never looked closely into problems
that depend on measurements of speed. [He strenuously
congratulates Debeaune on the quality of his work in thisarea, and in geometry and the physics of sound. Then:] All
that remains is for me to tell you (i) what it is that gives me
problems about speed and, connected with that, (ii) whatfrom Regius, 9.iii.39:
I don’t have words to express the joy that your admirable
letter of last August brought me. The modest reputation that
I already had (thanks to you and to Reneri) was enlarged bythat letter. It was indeed so much enlarged that my college
has attracted people—medical students, philosophers, legal
theorists, theologians, and others—who come to hear the
public and private lectures on medicine that I give following
the principles of your philosophy, which I have picked up
from your excellent works and viva voce from Reneri.
That should be enough, one would think, to raise my levelof courage, and also to open up the ways of nature to me.
Yet your goodness has led you to take other steps to help me.
You have allowed me, every time Reneri has visited you, to95

Correspondence

René Descartes

I think about the nature of weight, which you call ‘naturalinertia’.
(i) I hold that in the whole of created matter there is acertain quantity of motion which never increases or diminishes. When one body moves another, it loses as muchof its own motion as it gives to the other; thus, when astone falls to earth from a high place, if it hits the groundwithout bouncing, that’s because it jolts the earth and sotransfers its motion to it; but if the ·part of the· earth thatit moves contains a thousand times as much matter as thestone, when the stone transfers the whole of its •motion
to it, it passes along only one thousandth of its •speed. If
two unequal bodies receive the same amount of motion, thelarger one doesn’t get the same speed as the smaller. In
this sense, then, one can say that the more matter a bodycontains, the more ‘natural inertia’ it has. One can say toothat a large body is better able than a small one to transfer
its motion to other bodies; and that it’s harder for otherbodies to move a larger body than to move a smaller one. So
there’s one sort of inertia that depends on the quantity ofthe matter, and another that depends on the extent of its
surfaces. [The extent of a thing’s surfaces presumably = its size.](ii) Here is how I conceive weight. All the subtle matter
between here and the moon swirls rapidly around the earth,pushing towards the earth all the bodies that can’t move as
fast as it does. It pushes them with more force when they
haven’t yet begun to fall than when they are already falling;if they are falling as fast as the subtle matter is moving, it
won’t push them at all, and if they are falling faster than itis, it will actually hold them back.
So, you see, before reaching any conclusions about •speed
one has many things to think about; and that’s why I havealways held back from investigating •it; but these principles·of mine· enable us to explain many things that couldn’t

1638–1640

be explained before. I haven’t been willing to discuss these
topics elsewhere because the proof of them depends upon
my World; and I discuss them freely with you because I am
confident that you will view them in a favourable light.
L[6.v.39: Descartes writes to Pollot, saying that he would be glad andhonoured if Pollot paid him a visit but that he doesn’t ask for this because
he doesn’t think such a visit would be worthwhile from Pollot’s point ofview. Pollot has just sent Descartes a book, but Descartes won’t commenton it except viva voce when they next meet; because he doesn’t think wellof the book and doesn’t want to upset its author because he knows that
Pollot likes him. Sorrow over Reneri’s death. Sympathy with Pollot over
his recent time as a prisoner of war.]L[15.v.39: Huygens writes to Descartes, a friendly jokey letter urging

him to present his World to the world.]

L[28.v.39: Huygens writes to Descartes reinforcing his pleas for The

World to be published. Descartes will die some day—with jokes about
the spread of this bad habit of dying—and then if not sooner World will
be published, without Descartes there to answer honest people’s doubts
and refute the logical errors of malicious critics. ‘Don’t you care?’]L[6.vi.39: Descartes writes to Huygens, still declining to publish the

The World at this time, and speculating that Huygens’s plea is motivatedmainly by his thought that publishing the work would bring pleasure to
Descartes. As for his death: ‘I don’t think I need fear death for anotherthirty years unless it mounts a surprise attack.’]to Desargues, 19.vi.1639:
I have seen the frankness of your temperament, and have asense of my obligations to you, and these two factors leadme to write freely about your Treatise on Conic Sections, orabout what I guess to be in it judging by the Prospectus of
the work that Mersenne has sent me.
96

Correspondence

René Descartes

You could have either of two purposes in this book—purposes that require different means. One is(i) to write for the learned, the experts, teaching themsome properties of conic sections that they don’t yet
know about.
The other is(ii) to write for laymen, so as to make widely accessible (tothose who study your book) a range of things that until
now •have been understood only by a few and yet •arevery useful for perspective, visual art, architecture,and so on.
If (i) is your aim, I don’t think you need to introduce any newterms: the experts are used to the terminology of Apollonius,
and won’t easily switch to another even if it is better; yournew terminology will make your demonstrations harder tofollow and will discourage people from reading them.
If your aim is (ii) then your new terms—which are French,and have clearly been selected with intelligence and an eye tograce—will be better received by people whose heads aren’t
already full of the ancient terminology. And they may evenattract many people to the book—people who want to readabout coats of arms, hunting, architecture and so on, notaiming to be hunters or architects but wanting to be able totalk about such things using the proper terms. But if that isyour plan, you need to embark on a really big book in which
everything is explained fully and in such a vivid [see Glossary]and clear way that critics won’t be able to claim that the bookis ‘hard to understand’, by which they would mean ‘harderto understand that the description of an enchanted palace ina piece of romantic fiction’! (I’m talking about people—there
are plenty of them—who can’t study without yawning, who
can’t stretch their minds enough to take in a geometrical
proposition, and for whom it’s too much work to turn backa page or two to check letters ·in a demonstration· against

1638–1640

the diagram.) I think you would be helped in this if you
used the terms and calculations of arithmetic, as I did in
my Geometry, because many people who don’t know what
composition of forces is do know what multiplication is.
You treat parallel lines as lines that meet at an infinite
distance, so as to bring them into a single genus with pairsof lines that meet at a point. That is very good, provided thatin your hands the less obscure of these species [parallelism]
is used (as I’m sure it is) to clarify the more obscure one[infinity], and not vice versa. . . .

to Mersenne, 19.vi.1639:[This ten-page letter starts with comments on a perhapsmiracle that Mersenne has reported, saying that people
on the spot should have looked for evidence that it wasa miracle, because] why would God make a miracle if hedidn’t want people to know that that’s what it was?
[He then makes remarks about •the physics of flowing water, •subtle matter and the moon, •the advantages of lenses
over mirrors for magnification, •the hardness of ice, •subtle
matter and agitations—trembling hands, fire, etc., •a detailin the theory of looking-glasses, •a misunderstanding between Descartes and someone named Bessy (‘I interpreted
his proposition on the basis of his words, not his intention’);•geometrical work by Debeaune; half a dozen other episodes
involving half a dozen other people. Then:]
Concerning your remarks on weight: subtle matter
pushes the falling stone (i) around and also (ii) towards
the centre of the earth; but (i) is imperceptible because it iscommon to the whole earth and the surrounding air; so it
can only be (ii) that gives rise to weight. The stone moves
faster at the end of its descent than at the beginning, eventhough at that stage it is being propelled less forcefully by97

Correspondence

René Descartes

the subtle matter; that’s because this weaker impulse fromthe subtle matter is added to the impetus of the stone’s
preceding motion. Also: I did say that the subtle matter
revolves around the earth, but there was no need for me tosay whether it moves from east to west or vice versa, because
the motion can’t possibly be perceived by us. . . .
[Then further mopping-up operations, involving six more
people.]1638–1640

You experiment showing that water flowing from a 9-foottube must flow almost three times as fast as from a one-foottube strikes me as perfectly correct; though I add ‘almost’,
to take account of the air and of my view of the nature ofheaviness, according to which a body falling under its own
weight stops speeding up once it has reached a certain speed.
But I would like some time to go into the question of the
motion of water in greater detail; so I shan’t say anything
more about it here.
My conception of
how a candle flame or light from a glow-worm etc.
presses the subtle matter towards our eyes in astraight lineis the same as my conception of
how a stone swung round in a sling presses the pouchof the sling and pulls the cord in a straight line,
namely through the force of its circular motion. The subtle
matter around a candle or a glow-worm also moves in a circle·like the stone in the sling·, and tends to spread out fromthere leaving an empty space, i.e. a space containing only
what can enter it from outside. In the same way, we canconceive how subtle matter presses heavy bodies towards thecentre of the earth, simply by moving in a circle around theearth; and the earth doesn’t have to be at the centre of •theuniverse for this to occur. As long as it’s at the centre of •thecircular motion of all the subtle matter between us and themoon, that’s enough for it to make all the less subtle bodies
between us and the moon to tend towards the earth. . . .

to Mersenne, 27.viii.1639:
I was glad to learn of your return; I had started to worry
about your health, since I hadn’t received any news aboutyou. Two men you know died here recently, Heylichman andHortensius, not to mention my good friend Reneri, who diedlast Lent. You don’t need a war to find death.
I finally received the two copies of the book On Truth thatyou kindly sent to me [a French translation of De Veritate by Herbertof Cherbury]. When I can I’ll give one copy to Archbishop
Bannius on your behalf—I think that’s what you wanted. At
present I have no time to read; so all I can say is that when I
read the original Latin edition there were many things near
the beginning that I thought were very good; he was clearly
above average in his knowledge of metaphysics, a sciencethat hardly anyone understands. But later on he seemed tomingle religion with philosophy, which goes quite against thegrain with me; so I didn’t read it to the end, though I hope
to do this as soon as I can find the time to read anything. . . .
For the time being I’m studying without any book.
The twinkling of the stars may have to do with theliveliness of their light, which also makes them appear larger
than they are; but I offer several other explanations in myWorld.

L[ix.39: Descartes writes to Schooten about geometrical matters. He

hasn’t carefully studied what Schooten tells him about Debeaune’s Brief
notes on Descartes’s geometry because he is sure that that work won’t
have significant errors. Schooten’s difficulties with it, Descartes suggests, all come from Debeaune’s mis-labelling one line in a diagram—an‘excusable’ error.]98

CorrespondenceL

René Descartes

1638–1640[There follow two pages on a miscellany of minor mattersrelating to science or natural history, and a message to begiven to Descartes’s nephew if Mersenne should see him
again. Then:][x.39: Descartes writes to Huygens, three pages asking him to inter-cede with the Prince of Orange to get help for two Roman Catholic priests
who have a problem with the authorities of their own Church. Descartes
doesn’t know the details, but he vouches for the good character of these
two men (whom he has come to know through Huygens), and insists that

Since I last wrote, I have read the book you kindly sent me[Lord Herbert of Cherbury’s On Truth ]. Since you ask my opinionof it, and since it deals with a subject I have worked on all
my life, I think I should write something about it in this letter.
I find in it many good things, but they won’t please everyone,
because not many people can understand metaphysics. In
the general plan of the book the author takes a very different
route from the one I have followed. He examines what truth
is, and I have never had any doubt about that because it
strikes me as being such a transcendentally clear notionthat it’s impossible not to know it. There are many ways of
examining a balance before using it, but there’s no way to•learn what truth is if one doesn’t ·already· know it by nature.
When we •learn something, why would we accept the lesson
if we didn’t know it was true, i.e. if we didn’t know truth?
Of course we can explain the meaning of ‘truth’ to someone
who doesn’t know the language, telling him that ‘truth’, inthe strict sense, refers to the conformity of a thought with
its object, and that when something other than a thought
is called ‘true’—·e.g. ‘true gold’, ‘true courage’·—that means
only that the thing in question can be the object of true
thoughts, either ours or God’s. But no logical definition canbe given that would help anyone to discover the nature oftruth. I think the same of many other things that are very
simple and are known naturally, such as shape, size, motion,
place, time, and so on: if you try to define these things you
only obscure them and get into difficulties. For instance, aman who walks across a room shows what motion is better
than a man who says ·as Aristotle did· ‘It is the actuality ofa potential being in so far as it is potential’ , and so on.

helping them won’t produce any political fall-out.]

to Mersenne, 16.x.1639:[This 12-page letter starts with a few pages on hydrostatics[water in tubes etc.]; then lenses and light (how it is that light
can be reflected by a seemingly pure-black surface); thenthis response to Mersenne’s report that according to Mydorge
missile can eventually go infinitely fast:] You forgot to tellme whether this is supposed to happen in vacuo or not, so I
can’t refute it. All I can say is that the idea of some natural
thing’s going infinitely fast is self-contradictory, unless you
borrow from Desargue’s work (·in which he defines parallellines as lines that meet infinitely far off·) and say that an
ordinary motionless straight line AB is the same as a point
moving infinitely fast ·back and forth· between A and B. . . .
[A paragraph about how Descartes is currently assailed
from all sides: protestants hate him as a catholic, andcatholics don’t like him because he accepts ‘the hereticalview that the earth moves’. Then:]
To understand how subtle matter swirling around theearth drives heavy bodies towards the centre of the earth, filla round vessel with tiny lead pellets, and mix in some larger
pieces of wood or other material that is lighter than lead.
Now spin the vessel around very quickly, and you’ll see thatthe pellets drive the pieces of lighter material towards thecentre of the vessel, just as subtle matter drives terrestrial
bodies towards the centre of the earth.

99

Correspondence

René Descartes

The author takes universal consent as the criterion of
his truths; whereas my only criterion is the natural light [seeGlossary]. The two criteria agree in part: all men have thesame natural light, so you’d think they should have the same
notions; but there’s also a great difference between them,because hardly anyone makes good use of that light, which
is why it can happen that many people—perhaps everyone
we know—share the same mistaken opinion. Also there aremany things that can he known by the natural light but
haven’t yet been thought of.
He holds that we have as many faculties as there arevariations in objects of knowledge. This seems to me likesaying that because a piece of wax can take on an infinityof shapes it has an infinity of faculties for taking them on.
In that sense it is true, but this way of talking seems to mequite useless, and indeed rather harmful because it may
lead ignorant people to imagine a host of little entities—·little
things·—in our soul. So I prefer this way of thinking about it:•the wax, simply by being flexible, takes on all sorts ofshapes, and•the soul acquires all its knowledge by reflecting either
on itself (for intellectual matters) or on the various
dispositions of the brain it is joined to (for corporeal
matters)—dispositions that may be caused by thesenses or by other factors.
But it’s very useful not to accept any belief without considering what entitles us or causes us to accept it; and thiscomes to the same thing as his advice always to consider
what faculty one is using, etc.. . . .
He recommends that one should above all follow natural
instinct, from which he derives all his common notions [seeGlossary]. For my part, I distinguish two kinds of instinct.
There’s the one we have because we are human beings, which
is purely intellectual: it is the natural light or mental vision.

1638–1640

I hold that this is the only instinct we should trust. The otherbelongs to us because we are animals; it’s a certain natural
impulse towards the preservation of our body, towards theenjoyment of bodily pleasures, and so on. This should notalways be followed. . . .
What he says about religion I leave to be examined by
the gentlemen of the Sorbonne. I can only say •that I foundit much easier to read in French than I did before in Latin;•that he has many maxims that seem to me so pious, and somuch in conformity with common sense, that I hope they’ll
be approved by orthodox theology; and •that although I can’t
agree with all the opinions of this author, I regard him as aperson of quite extraordinary talent.

to Mersenne, 13.xi.1639:[Descartes discusses •different pumping arrangements toraise water more than 100 feet; •reflections from blacksurfaces; •how a missile would move in empty space if there
any; •getting seeds for ‘sensitive plants’ and exchange ofgarden catalogues; •someone’s accusation that Descartes ismoving towards being a Calvinist (indignantly denied, witha full page of evidence; and •the way a person in a painting
seems to be looking straight at you when you move from one
side of the picture to the other. Then:]
The opinions of your analysts—·Roberval and other geometers of Paris·—about the existence of God and the honour
that is due to him, are as you say very difficult to cure;not because of any shortage of reasons strong enough toconvince them, but because people like that who are convinced of their own intelligence are often less capable ofreasoning than others. The part of the mind that most helps
in mathematics, namely imagination, hinders more thanit helps in metaphysical speculation. I am now writing a100

Correspondence

René Descartes

discourse in which I try to clarify what I have previously
written on this topic. It will occupy only five or six printed
sheets, but I expect it to contain a great part of metaphysics.
As a way of improving it, I plan to have only 20 or 30 copies
printed, and to send them to the 20 or 30 most learned
theologians I can find, so as to get their judgment on it andlearn what should be changed, corrected or added before
publication.
I believe that in empty space—if such a thing werepossible—a very small force could move an enormous bodyjust as it could move a tiny one, though not at the same
speed. A force that could move a 10lb rock at a certain speed
could move a 5lb rock at twice the speed.
We aren’t prevented from throwing a stone very high by(a) the cross-current of swirling subtle matter that it goes
through. There’s nothing surprising in that. Our arm inthrowing gets its force from (b) a still faster torrent of subtle
matter—the one that activates our animal spirits, and differs
in force and causal energy as much as fire differs from air.
[The letter ends with scepticism about what Mersenne
inferred from an experiment he performed, relating •rateof water-flow to •diameter of tube; and with anxiety about
Mersenne’s plan to visit Italy, ‘which is a very unhealthy
country for Frenchmen’.

1638–1640(2) This subtle matter puts limits on how high we can•throw a stone or •jump; for if this matter didn’t push heavy

bodies down again, when we threw a stone high up it would
keep on going, and so would we when we jumped upwards.
[(3) Descartes says about inertia pretty much what hesaid to Debeaune on 30.iv.30 [page 95].](4) I’m not surprised that some people can demonstrate
theorems on conic sections more easily than Apollonius
could; his demonstrations are extremely long and tangled,
while their conclusions, considered in themselves, are fairly
simple. But we can look to conic sections for other resultsthat couldn’t be easily untangled by a 16-year-old. [That was
Blaine Pascal’s age at that time; Mersenne had told Descartes about him
in a letter of 12.xi.39, which we don’t have.](5–7) Everyone’s desire to have every perfection he canconceive of, and consequently all the perfections we believe
God to have, is due to God’s having given us a will thathas no limits. It is principally because of this infinite [here =‘unlimited’] will within us that we can be said to be created in
his image.
[In (8) Descartes criticises a certain explanation of why aman immersed in water doesn’t feel the weight of the water.
In (10) he explains why we go upwards when we jump, andconnects this with an account of how birds fly.](10) I have noticed that Lord Herbert of Cherbury treats
as common notions many things that aren’t. It is certain that
nothing should be taken as such unless it can’t be denied
by anyone.
I turn to your letter of 4.xii and thank you for your advice
about my Essay on Metaphysics. The arguments of RaymondLull are all invalid; I don’t take them seriously. As for theobjections of your analysts: I’ll try to answer them without
expounding them. That is, I shall present the foundations
from which •those who know the ·analysts’· objections canto Mersenne, 25.xii.1639:
I owe a reply to three of your letters, namely those of 12.ix,
4.xii and 10.xii; the last two arrived on the same day.
(1) You ask me why a bow or a spring loses its force whenstretched wide for a very long time. This is easy to explain interms of my principles. The pores that I earlier said have an
oval shape gradually become round, because of the particlesof the subtle matter that continually flow from them.
101

Correspondence

René Descartes

to Mersenne, 29.i.1640:

derive their refutation, while •those who don’t know theobjections won’t learn them from me. I think this is how one
should treat the matter. And I’m not so short of books as
you think; I have here Aquinas’s Summa Theologica and a
Bible that I brought from France.

[Mersenne has often complained about the conduct of aman named Rivet; Descartes said in an earlier letter that
he was tired of this topic; but he puts it at the start of thisletter because, he says, Mersenne put it at the start of his
most recent letter (which we don’t have). •Descartes givesa three-page narrative about how Rivet was publicly shown
up as a charlatan who knows little except some tricks fordeceiving people. •A recent English book on magnetic declination doesn’t amount to much; ‘it offers three observations
in support of its conclusion; I want thousands of them’. Also
brief remarks about •other people and bits of science. Then:]I have just re-read my notes on Galileo, where I didn’t
actually say that •a falling body passes through every degreeof slowness; but I did say that •this can’t be settled until
we know what heaviness is, which comes to the same thing.
I agree that your example of the inclined plane proves that all
speed is infinitely divisible; but I don’t agree that when a body
starts to fall it passes through all these speeds. I don’t think
you suppose that a ball struck by a mallet starts moving
more slowly than the mallet does!. . . . In my view, all there
is to heaviness is the fact that terrestrial bodies are reallypushed towards the centre of the earth by subtle matter—andyou can easily see what follows from this. But don’t inferthat when these bodies start to move, they immediately
move as fast as this subtle matter; for it pushes them only
obliquely, and their speed—especially that of the lightest
ones—is considerably reduced by the air.
I’m surprised that you hadn’t heard that it’s easier tohammer a lump of lead flat when it is resting on a cushion(or an anvil suspended so that it can move when struck)than when it rests on a rigidly fixed anvil. This is a matterof common knowledge; there are countless facts like it in[Descartes now has eight numbered items, replying toMersenne’s second letter: the mechanics of hammer-blows,
techniques for raising water high, black bodies, the speed offalling stones, the offer of seeds and a garden catalogue, whatmust be wrong in Mersenne’s experiments on water-flow,
forwarding-addresses, and this:] Thank you for the affection
you show me in planning to take some of my letters to you
with you when you go to Italy; but I don’t think that anything
there is worth showing to anyone ·else·. Let me explain. I
have often given you my opinions on matters that I haven’t
thought about before writing to you about them; and having
sometimes had to respond to you on 20 or 30 different topics
in an evening, I couldn’t possibly think well about all ofthem.
[In 14 numbered items in response to Mersenne’s third
letter, Descartes addresses the speed of falling water, in tubes
and in rivers; subtle matter and weight, and our bodies;things that do/don’t move with the water they are floating in;determining the height of mountains; compasses reading differently in different countries; the intellectual misconduct of‘your geometers’; Debeaune’s Notes on Descartes’s Geometry(he is free to publish them if he wants to).]L

1638–1640[28.xii.39: Huygens writes to Descartes about his attempt to get asome battling mathematicians to sign a ‘Compromise’ document thatwould enable the battle to stop. At the center of the fight has been
Stampioen, who refused to sign and treated Huygens discourteously.
Huygens is disgusted with the whole affair. There’s more about this on
page 104 below.]102

Correspondence

René Descartes

mechanics, all explained in the same way. To flatten a lumpof lead, you need not only to •strike it with great force but to•continue that force for long enough to give the lead-particles
time to change their positions. When the lead is on a fixed
anvil, the hammer bounces back up at almost the moment
when it strikes, so that it has less time to flatten the leadthan it would if the lead were on a supporting body that
could give way to the blow, thus allowing a longer period ofcontact between mallet and lead.
[The letter continues, addressing topics raised by
Mersenne: •the physics of falling bodies, •the physics ofcollisions, •black surfaces, •bending several bows at once,•hoisting water. Then, ‘so as to give you some news’,
Descartes reports on a recent wind-storm with strange
effects; on the sudden disappearance of a sand-bank, tothe advantage of a seaside town in Zealand; and on this:]
When Hortensius was in Italy a few years ago he found out
how to make horoscopes. Back in this country he told two
young acquaintances that he would die in 1639 and theywouldn’t live long beyond that date. Now, as you know he
did die last summer, and this struck fear into the two young
men—so much so that one of them is dead and the other(who is Heinsius’s son) is so sad and so languishing that heseems to be doing his best to save astrology from being aliar. What fine science that is—bringing death to people whootherwise might not even have been ill!. . . .

1638–1640

join wholeheartedly in it that those who do should especiallyvalue each other and try to help each other by sharing
their empirical data and their thoughts; and I offer you
this co-operation, with every kind of affection.
To make a start I’ll answer in this letter the question you
asked me the function of the pineal [see Glossary] gland. I holdthat this gland is the principal seat of the soul, and the place
where all our thoughts are formed. I base this belief on thefact that this is the only part of the brain that isn’t double.
We see one thing with two eyes, and hear one voice with twoears, and in short never have more than one thought at atime; so it must be the case that the species [see Glossary] thatenter by the two eyes or by the two ears etc. come together insome ·one· part of the body where the soul can take accountof them. The only such place in the whole head is this gland;
and it’s situated in the best possible place for this purpose,
namely in the midst of all the concavities; and it is supported
and surrounded by the capillaries of the carotid arteries thatbring the ·animal· spirits into the brain. As for the species
preserved in the memory, I imagine them as being like thefolds that this paper retains after being folded; so I think thatmost of them are held in the whole substance of the brain,
though some of them may also be present in some way inthis gland, especially in people whose minds are sluggish.
In the case of very good and subtle minds, I think the glandmust be free from outside influence and easy to move, as
witness the fact that the pineal gland is smaller in man thanin ·other· animals—the reverse of what holds for the otherparts of the brain. I believe also that some of the species
that serve the memory can be in various other parts of the
body: the skill of a lutenist, for example, is not only in hishead but also partly in the muscles of his hands. As for the
likenesses of tiny dogs that are said to appear in the urine ofthose who have been bitten by mad dogs, I must admit thatto Meysonnier, 29.i.1640:
I would have written to you first if I’d known you to besuch as you describe yourself in the letter you have kindly
written to me [a letter that we don’t now have]; for the search
for truth is so essential and so daunting that it needs the
co-operation of many thousands of men; and so few people
103

Correspondence

René Descartes

I have always thought it was a fable, and I’ll go on finding
it hard to believe in them unless you tell me that you haveseen very distinct and well-formed examples of this. But if
this really does happen, it might be explained somehow, likethe explanation of the birth-marks that children receive fromthe cravings of their mothers.
L

1638–1640

It’s not right to be wholly ignorant of what the books contain,
but you never need more than the general acquaintancethat is an automatic by-product of whipping through theprincipal authors. This will let you identify the passages
where you can look up previous discoveries when you needthem. Many things are much better kept in books thanmemorised—astronomical observations, tables, rules, theorems, in short anything that doesn’t automatically stick inthe memory at the first encounter. The fewer items we load
onto our memory, the better equipped our mind will be toincrease its knowledge.
It would be an excellent thing if the historical part ofmathematics, which is scattered among many volumes andis still a work in progress, were all collected within a single
book. This wouldn’t involve expenses for finding or buying books, because there has been a great deal of mutual
copying of material among the relevant authors, and there’s
nothing anywhere that can’t be found in any moderately
adequate library. What would mainly be needed is not
so much •diligence in collecting everything as •judgement
in rejecting what is superfluous, and •knowledge [scientia ]to supply material that hasn’t previously been discovered.
And the only person who has all these qualities is your‘self-sufficient’ mathematician. If such a book did exist,
anyone could easily learn from it the whole of mathematical
history and even a part of mathematical science [= could learn[1.ii.40: Descartes writes to Waessenaer a complicated letter in which

he is trying to weigh in on Waessenaer’s side in a long-running disputewith Stampioen concerning the value, and also the integrity, of certain
mathematical work. At issue also is the view that faking results in mathematics should be legally treated as a worse crime than counterfeiting
money.]

to Hogelande, 8.ii.1640:[Descartes says that he recently returned a pamphlet and abook, both on mathematics, that Hogelande had sent him,and explains why he didn’t read the pamphlet carefully when
he had it. He does remember that there was nothing init that he disagreed with much, and he approves of its
general approach, which he says shows the author to be‘self-sufficient’ [each occurrence of this term is given in Geek]. Then:]I generally distinguish two parts of mathematics:•the historical part, consisting of everything that has
already been discovered and is contained in books;•the scientific part, i.e. the skill to solve every problem,
and thus to discover by one’s own efforts everythingthat could be discovered in that science by means of
our native human intelligence.
Anyone who has such science certainly doesn’t need muchoutside help, and so counts as genuinely ‘self-sufficient’.

all of mathematics that is so far known, and even learn a bit about how
to do mathematics’].

But no-one will ever emerge as a truly‘self-sufficient’ mathematician unless he is also naturally
endowed with an intellectual aptitude for the subject, andhas then refined it by a long course of study.
So much for theoretical mathematics. As for its practicalapplication, if anyone wanted to possess everything relevantto this—instruments, machines, automata, and so on—he[Descartes here uses ‘science’ (in his Latin, scientia) in each of twosenses—‘in that science’ speaks of a discipline or departmentof knowledge, whereas ‘has such science’ is about knowledge.]

104

Correspondence

René Descartes

could never succeed, even if he were a king, by spending all
the treasure in the world. Anyway, there’s no need for all
this; it’s enough to know the descriptions of these things sothat when there’s a need we can make them ourselves or
have them made by craftsmen.

1638–1640

Mersenne about this:] After thanking him for his kindness,
say this to him. I don’t altogether deny that the species [seeGlossary] that serve memory may be partly in the pineal gland,
especially in lower animals and in humans who have coarse
minds. But it seems to me that other people wouldn’t beable easily to imagine countless things that they have neverseen, if their souls weren’t joined to some part of the brainthat was just right for •receiving all kinds of new impressions
and consequently no good at •storing them. This part of thebrain has to be the pineal gland, because it’s the only thing
in the whole head that isn’t double. But I think that it’s
the other parts of the brain—all of them, but especially theinterior parts—that provide most of the material for memory.
And all the nerves and muscles can also come into it: a lute
player, for instance, has a part of his memory in his hands;
the ease of various movements and positions of his fingers,which he has acquired by practice, helps him to remember
musical passages where these ·movements and· positions
come into play. You’ll find this easy to believe if you bearin mind that what people call ‘local memory’ is outside us:
for instance, when we have read a book, not all the species
that can remind us of its contents are in our brain. Many ofthem are on the paper of the copy we have read. It doesn’t
matter that these species are not like the things they remind
us of; the same is true of many of the species ·stored· in thebrain. . . . But in addition to this memory that depends on
the body, I recognise another sort of memory—intellectual
memory—which depends entirely on the soul.

to Mersenne, 11.iii.1640:[Descartes responds to things in several of Mersenne’s letters, concerning •collisions and the flattening of lead balls,•tempering steel, •the speed of falling bodies, •the speed of
missiles, and then:] I would think I knew nothing in physics
if I could say how things can be but couldn’t demonstratethat they can’t be otherwise. Such demonstrations are
perfectly possible once physics has been reduced to thelaws of mathematics. I think I can provide them for the small
area to which my knowledge extends; but I didn’t do themin my Essays because I didn’t want to present my principles
there—and I still don’t anything to persuade me to present
them in future.
[Then •Descartes’s rejection of the distinction between
natural and violent [see Glossary] movements; •seeds and catalogues; •Mersenne’s law-suit against Rivet (still sub judice,
Descartes reports, but near to completion); •the convulsionsof a nun (not miraculous; Descartes thinks he could cure
her, but he would have to see her first); •the weather; several
other small topics.][Descartes then declares that he’s not surprised that thepineal gland is hard to recognise in autopsies of humans; it’s
because the gland has time to decay during the days when
the investigating scientist attends to the intestines and otherparts before opening the head. Then:] The mobility of thisgland is sufficiently shown by where it is: it is supported

to Mersenne, 1.iv.1640:[This letter starts with a discussion of reports from Englandof changes in the direction of compass-needles: Descartes
doesn’t think these changes are caused by any big change
in the earth itself. Then, for Meyssonnier, who has asked
105

Correspondence

René Descartes

only by the little arteries that surround it, so it won’t takemuch to move it—but not, I think, to move it far in any
direction.
[Remarks on •the newborn child’s birth-marks indicating
the mother’s cravings, and signs in a lunatic’s urine of what
his mental condition is; •a passing jibe at Petit; •seeds andcatalogues; then:]
So you have had a letter from England indicating that I
was about to receive an invitation to go there. I have had noword of this myself; but I tell you in confidence that I would
prefer that country as a homeland to many others; and whenit comes to religion the King himself is said to be Catholicby choice; so please don’t discourage the good intentions ofyour correspondents. . . .
Lleft was less at fault; but now I fear that you may haverefrained from criticism because too much needs correction,or because the whole thing needs to be cancelled.
Now for your objections. In your first you say:
‘It is because we have some wisdom, power andgoodness that we form the idea of an infinite—or atleast indefinite—wisdom, power, goodness and theother perfections that we attribute to God; similarly
with our idea of infinite quantity’.
I entirely agree; I’m convinced that our only idea of God is
the one formed in this manner. But the whole point of myargument is this:
These perfections are so slight in me that my nature
couldn’t enable me to extend them in thought to an
infinite degree unless we derived our origin from abeing in which they are actually infinite.
Just as I couldn’t conceive of an indefinite quantity by looking
at a very small quantity or a finite body unless the world
actually was or at least could be indefinitely large.
In your second objection you say: ‘The truth of axioms
that are vividly and clearly understood is self-evident.’ I agree
that this is so while they are vividly and clearly understood,
because it’s a basic fact about our mind that it can’t help
assenting to what it clearly understands. But because weoften remember conclusions that we have deduced from suchpremises—remembering them while not actually attending
to the premises themselves—I say that on such occasions if
we don’t know God we can have this thought:‘Those conclusions are uncertain; I remember deducing them from clear principles, but perhaps my nature
is such that I go wrong even in the most evident
matters; in which case even at the moment when
I deduced them from those principles I didn’t outrightknow them but was only convinced of them.

[3.1v.40: Descartes writes to Golius, complaining of his slowness inrendering judgment on the Stampioen-Waessenaer affair, and reportingthat there are many malicious rumours in the air that won’t be clearedaway until Golius settles the dispute.]L

1638–1640[7.v.40: Descartes writes to Pollot, enclosing two books by Waessenaer

and a writing in which Stampioen first attacked Waessenaer without any
provocation. The piece by Stampioen contains a promise of a mathematical feat which, Descartes says, ‘is no more possible than whitening a
Moor’. With that warning given, this work ‘isn’t worth the time of anyone
who isn’t interested in the mœurs [see Glossary] of this man’.]to Regius, 24.v.1640:
I am much obliged to you and Emilius for examining andcorrecting the manuscript ·of the Meditations· which I sentyou. I see that you were even kind enough to correct thepunctuation and spelling. You’d have put me under an evengreater obligation if you had been willing to make changes
in the words and the thoughts. However small such changes
were, they would have given me hope that what you had
106

Correspondence

René Descartes

to Mersenne, 11.vi.1640:

I distinguish these two as follows: when you are only convinced that P, there remains some reason that might lead you
to doubt whether P; but you know that P if your conviction isbased on a reason so strong that it can never be shaken by
any stronger reason. Nobody can know ·in this sense· unlesshe also has knowledge of God. But once you have clearly
understood the reasons that convince us that God exists andisn’t a deceiver, provided you remember the conclusion ‘God
is no deceiver’ you will continue to know this and not merely
be convinced of it—and the same holds for all the otherconclusions the reasons for which you remember having
once clearly perceived.

[This letter has about a dozen pages on miscellaneous scientific matters that Mersenne has written about, mostly ones
that have figured in several earlier letters by both men. Two
separate episodes in this material are worth recording here:]. . . .You ask (on behalf of Desargues) how the hardnessof bodies can come purely from the motionlessness of theirparts. To understand this matter you have to take in that(a) a body’s movement
is different from(b) its determination to be moved in one direction rather
than another;and that force is needed only for (a), not for (b); because (b) depends less on any (b)-force than on how this body’s (a)-force
is situated in relation to the (a)-forces of the surrounding
bodies. And you need also to see that there is no vacuum innature, and no rarefaction and condensation of the sort thatphilosophers describe. [That is, it never happens that the very same

In your latest objections—they arrived yesterday andreminded me to reply to your earlier ones—you say: ‘Allrashness of judgement depends on some state of the body,whether innate or acquired.’ I flatly disagree. That would
take away the freedom and scope of our will, which canremedy such rashness. If it doesn’t remedy it, the resulting
error is a privation [see Glossary] in relation to us but a mere
negation in relation to God.

portion of matter occupies different amounts of space at different times.]

What actually happens when a body is rarefied is that someother more subtle matter enters its pores, etc. It follows
from this that no body x1 can be moved without displacingsome other body x2 at the same instant, with x2 displacing
a third body x3 at that instant, and so on until body xn−1
displaces xn which enters the space that x1 is leaving. So
that no body can move unless a complete circle—·or, anyway,
a closed ring·—of bodies moves at the same time. It’s also
important that any body—even one moving in a circle oralong a curve—tends to continue moving in a straight line;
you see this when a stone whirled around in a sling flies
straight when it is released from the sling.
. . . .There’s no doubt that the folds of the memory get in one
another’s way, and that there can’t be an infinity of such

[There are four more pages, commenting on some theses
which Regius was to present and defend in public quite
soon. Descartes offers to come and witness this occasion
from a viewpoint where no-one would recognise him. One
comment includes a bit that might interest us:] I don’t see
why you think that the perception of universals belongs to
the imagination rather than to the intellect. I hold that thisactivity of relating a single idea to many things is performedby the intellect alone.
L

1638–1640[30.v.40: Regius writes to Descartes, saying that he has modified

his theses in the light of Descartes’s comments, and reporting that thejudges in Leiden have come down in favour of Waessenaer and against
Stampioen, who has been ordered to donate 600 pounds to the poor.]107

Correspondence

René Descartes

folds in the brain; but they are quite numerous. And theintellectual memory has its own separate species [see Glossary]
which don’t depend at all on these folds. So I don’t believethat the number of folds has to be very large.
I don’t explain the feeling of pain without reference to thesoul. According to me, pain exists only in the understanding. I do explain all the bodily movements that accompany
this feeling in us; in ·non-human· animals it’s only these
movements that occur, and not pain in the strict sense.
Llife of a fetus relate to events in the life of the pregnant
mother. Then:]
As for brute animals, we’re so used to thinking they havefeelings as we do that it’s hard to think otherwise. But
suppose the following were the case:
There are automata that perfectly imitate every one of
our actions that such a machine could imitate; and wenever take them to be anything more than automata.
If that were so, we would have no doubt that all the animals
that lack reason are automata too, because those animals
differ from us in exactly the way the automata did. In my
World I explain in great detail how the bodies of animals
contain all the organs that an automaton would need if it
was to imitate those of our actions that are common to us
and the beasts.
[A paragraph commenting on various anecdotes concerning medical anomalies. Then:] The letter from Villiers
contains no argument to refute what I have said about thepineal gland, except that it can alter, like the rest of the brain.
That is no reason why it can’t be the principal seat of thesoul; for the soul certainly must be joined to some part of thebody, and the pineal gland undergoes less alteration thanany other part of the body. Although it is very small and soft,
it is in such a well-protected place that it’s almost immune
from illness, like the lens of the eye. It happens much more
often that people become troubled in their minds without any
known cause—which could be attributed to some maladyof this gland—than it happens that sight is lost through amalady of the lens. Moreover, all the alterations that occur
in the mind when a man sleeps after drinking or the like canbe attributed to some alterations occurring in this gland.
He says that the soul can make use of double parts(I agree) and can use the ·animal· spirits, which can’t all
reside in the pineal gland. I agree with that too, because[13 and 24.vi.40: Descartes writes twice to Wilhelm about the contin-uing health problems of the latter’s daughter, offering all possible help
from Descartes’s friend the physician Hogelande.]L[22.vii.40: Descartes writes to Mersenne thanking him for his support

in the matter of some anti-Descartes ‘Theses’ adopted by the Jesuit order.
He isn’t surprised to be told that their author (Bourdin) is a relative of
Petit, and expresses contempt for Bourdin’s ‘Confrontation’ (Velitation)that appears as their preface.]L[22.vii.40: Descartes writes to Hayneuve, ‘humbly’ asking for correc-

tions of any errors he has committed, and saying that nobody could dothis better than the Jesuits. He asks to be shown all their reasoning thatconflicts with things he has written.]L

1638–1640[29.vii.40: Descartes writes to Mersenne, enclosing a dozen pagesof Latin addressed to Bourdin, replying to his attack mentioned twoparagraphs back.]

to Mersenne, 30.vii.1640:[Descartes refers to some views of Meysonnier’s that
Mersenne has sent him, saying that some of them] arewell above my head, i.e. (between ourselves) they seem tobe unintelligible. Then a paragraph of speculation aboutbirth-marks and how they might be cured, all based on aview that now seems merely weird, about how events in the108Correspondence

René Descartes

I don’t think that the soul is so imprisoned in the gland thatit can’t act elsewhere. But using a thing isn’t the same as
being immediately joined or united to it; and since our soulis single and indivisible, it seems to me that the part of the
body it is most immediately joined to must also be single andnot divided into a pair of similar parts. The pineal gland is
the only thing in the entire brain, so far as I can discover,that is single in this way. . . .
[The letter continues by •listing and dismissing other candidates for the role of brain-singletons; •sharp comments on
Villiers’s idea of ‘inert spirit’, which Descartes compares with‘shadowy light’ and ‘hard liquid’; •a suggested explanation ofwhirlpools; and then:]I haven’t yet had my five or six sheets of metaphysics
printed, though they have been ready for some time. [Thisis the Meditations.] I delayed because I don’t want them tofall into the hands of pseudo-theologians—or, now, into
the hands of the Jesuits whom I foresee I shall have to
go to war with—before they have been read and approvedby various learned men and if possible by the Sorbonne asa whole. I intended to travel in France this summer, andplanned to take them there myself; and I didn’t want to havethem printed until I was about to depart, for fear that the
publisher would—publishers do—steal copies to sell without
my knowledge. But the summer is already so far gone that
I fear I won’t be able to make the journey. In that case I’ll
send you ten or twelve copies, or more if you think they will
be needed. I will have printed only as many as are needed forthis purpose, and I will ask you to distribute and guard them.
Please give them only to the theologians you consider to be
the most able, and the least prejudiced by (and committedto) scholastic errors—really good people who are moved moreby truth and the glory of God than by envy and jealousy.

1638–1640

I am scandalised by Bourdin’s ‘Confrontation’ [see first
letter on 22.vii.40], because he doesn’t oppose anything that I

have actually said, but represents me as saying stupid things
that I have never thought, and then goes on to refute them.
[Descartes goes on to say that he will in due course publicly
answer Bourdin’s attack; he clearly enjoys the thought ofBourdin’s humiliation. He then replies to three of Bourdin’spoints, and to some of Mersenne’s. Then: a report ofiron apparently suspended in the air by a single magnet(Descartes suggests that a silk thread was used); discussionof the three basic elements according to the ‘chemists’ (here= alchemists); the flow of water; weight; subtle matter.]to Huygens, 31.vii.1640:
I’m surprised that you have been told that I was going to
publish something on metaphysics, because I haven’t yet
delivered anything to the publisher, or indeed fully preparedanything that isn’t too slight to be worth mentioning. In
short, what you have been told about this must be quite
inaccurate—apart from what I told you last winter, namelythat I was proposing to clarify what I wrote in Part Four of
the Discourse on the Method, not to publish it but merely
to have a dozen or so copies printed to send to leading
theologians for their verdict. ·To see what I am up to·,compare my work in this area with the demonstrations ofApollonius. Everything in these is very clear and certain,
when each point is considered separately; but the proofs
are rather long, and the necessity of the conclusion can’t
be seen unless one remembers exactly everything that hasgone before; and that’s why you’ll hardly find a single person
in an entire country who can understand them. And yet,because the few who do understand them vouch for theirtruth, everyone believes them. Similarly, I think I have fully
109

Correspondence

René Descartes

demonstrated the existence of God and the non-material
nature of the human soul; but I do this through a seriesof linked arguments, and anyone who forgets the smallest
detail won’t be able to understand the conclusion. So myarguments won’t bear much fruit unless I reach readers
who are highly intelligent and enjoy a high reputation in thefield of metaphysics; if they take the trouble to examine myarguments with care and state frankly what they think ofthem, they’ll encourage the rest to follow their judgement—or
at least make them ashamed to contradict them without
reason. Moreover, since this treatise concerns the glory ofGod, I think I am obliged to take more care to do it justicethan I’d be disposed to take if it concerned some other topic.
I think I’m about to go to war against the Jesuits. Theirmathematician in Paris [Bourdin] has publicly attacked my
Optics in his theses, and I have written to ·Hayneuve·, his
superior ·in the Society of Jesus·, with a view to involving thewhole Society in this quarrel. I have known for years that it’sbetter not to stir up adversaries, but ·I make an exception ofthis case·: they will be angry with me whatever I do, and I
can’t avoid this anger; so I think it’s better to face them all in
one big battle rather than waiting for individual skirmishes
that would go on for ever.
I am planning to visit France—for five or six weeks if I canmanage that—to deal with family affairs. But Waessenaer
doesn’t want me to leave before the publication of a thing hehas been forced to write by the stubbornness of his opponent·Stampioen·; and though I am thoroughly sick of this battle,
honour requires me to see it through to the end, and my duty
to this country requires me to speak the truth openly. You’ll
see the truth in Waessenaer’s preface; and I’m willing to delay
the printing of it for two weeks (or more, if necessary) in orderto get your judgment on it if you would be so good as to send
it to me. If and when you do, we—·Waessenaer and I·—will

1638–1640

treat your judgment as an unbreakable law. In the meantime,
I solemnly assure you that •even before Stampioen went into
print he knew that his book was worthless, as can be seenfrom the tricks he played; and that •he has the ‘wisdom’ of
Socrates, in that he knows that he doesn’t know anything,
but has incredible impudence when it comes to blackening
someone’s name with lies and boasting of his knowledgeof things that are impossible and extravagant. This lastis the most dangerous and damaging quality for a man in
his position—·i.e. for a young academic who isn’t yet on asecure career-path·; and I think I ought to tell you what myjudgment of him is.

to Mersenne, 6.viii.1640:
I left myself so little time to write to you a week ago that I
didn’t have time to answer all the points of your last letter,and I stopped at the one about the folds of memory. I don’t
think that our memories require a vast number of these folds,
because a lot of things that resemble one another are served
by a single fold. Also, in addition to the bodily memory whose
impressions can be explained by these folds in the brain,
I hold that our intellect has another sort of memory that is
altogether spiritual [here = ‘mental’]; it is what we mainly use,and non-human animals don’t have it.
It’s a mistake to believe that we remember best what we
did when we were young. Back then we did countless thingsof which we no longer remember anything. And when we
do remember something from our early years, that’s not
only because of impressions that we received back then, but
also—and mainly—because we have had those memories
before and have renewed the impressions by remembering
the events at various times since.

110Correspondence

René Descartes

L

As for the tides: this is something that depends entirely
on my World, and I can’t make a good job of explaining it
separately; but I can’t refuse you anything, so I’ll try to givea rough account here. [Descartes’s account, accompanied
by a diagram, is essentially this. The earth is surroundedby ‘the heaven’ [see Glossary], which is a fluid rotating aroundthe earth and keeping it in place. The moon rotates too, butmore slowly than the heaven, with the result that wherever
the moon is at any given time the downward pressure of
the heaven is a bit stronger than in other parts of the circle;
so the earth is always being slightly pushed away from themoon, and that slight movement by the earth somewhat
flattens the oceans on the side towards the moon and on theexactly opposite side, making the water rise a little on theother two sides. The result is the tides. Descartes adds an
explanation of why the interval between two tides is slightly
less than 12 hours. Then:]Also, as I report in my World the heaven can’t be exactly
circular but must be slightly oval, and the moon is situatedon the smallest diameter of the oval when it is full or new,which explains why the tides are bigger than usual at thesetimes. The tides are also affected by variation in the shapeetc. of the coastline. I would prefer this account of the tides
not to be published or widely circulated, because it’s a partof my World, and if the book ever sees the light of day I would
like it to retain some novelty value.
[The remaining three pages touch on an anecdote abouta magnet in England (‘fable’, says Descartes); the questionof where in its flight an arrow is at its maximum speed;
thoughts about how he will respond to various physicians
whose views on his work Mersenne has reported; and an enclosure that Descartes asks Mersenne to show to anyone who
is disposed to take seriously the output of an ‘impudent liar’
named Rivet [see first paragraph of 29.i.1640 letter to Mersenne].]1638–1640[14.viii.40: Huygens writes to Descartes, explaining his lateness in

replying (he had to move with the army), and approving Waesennaer’s
not-yet-published Preface (see last paragraph of Descartes’s 31.vii.1640
letter to Huygens). Apology for the misunderstanding about Descartes’s
publication plans (see opening of that letter), and speaks of how heand others are ‘hungry’ for more of Descartes’s work. Comments on
Descartes’s plan to visit France, and on the activities of that ‘stupid boy’
Stampioen.]L[17.viii.40: Descartes writes to Wilhelm about the pay-off from Stam-pioen’s losing his bet against Waessenaer (see Regius’s 30.v.40 letter toDescartes). The money had been held by the Rector of Leiden university,
Dedel; in the event of Stampioen’s losing, the money was to go to thepoor of the town; but Dedel has given it all to one hospital that was built
by the rich people of Leiden; and, rich or poor, it shouldn’t have been
handed out without consulting Waessenaer.]L[viii.40: Descartes writes to Huygens, welcoming and praising a pam-phlet in Flemish by Huygens on the use of organs in Dutch Churches;and making good-humoured comments on some of the mildly disparaging terms that the pamphlet uses in referring to Roman Catholics.]

L[30.viii.40: Descartes writes to Mersenne about a variety of scientificmatters, also commenting on the ‘theses’ that the Jesuits have issued
against him (see the second paragraph of Descartes’s 31.vii.1640 letterto Huygens). These, he says, are entirely Bourdin’s work, though otherJesuits have also spoken against him.]L[30.viii.40: Descartes writes to Mersenne again on the same day,this time in Latin. The letter is a formal response to the anti-Descartes‘theses’ of the Jesuits.]

to Mersenne, 30.ix.1640:
There’s something on which I would be glad to have youradvice and information. As I told you, I intended to haveprinted only 20 or 30 copies of my little treatise on meta111

Correspondence

René Descartes

physics [the Meditations], and to send them to 20 or 30 theologians for their opinion of it. But I don’t see that can be done
without the book’s being seen by almost everyone who has
any curiosity to see it; they’ll borrow it from one of thoseto whom I send it, or get it from the publisher (who willcertainly print more copies than I order). So perhaps I’ll
do better to have a public printing of it from the start. I’m
not afraid that it contains anything that could displease thetheologians; but I would have liked to have the approval of anumber of ·learned· people so as to prevent its being picked
at by ignorant contradiction-mongers. The less such people
understand it, and the less they expect the general public
to understand it, the more eloquent they will be—unless theauthority of a number of learned people holds them back.
With this in mind, I thought I might send you my treatise
in manuscript for you to show to Father Gibieuf, and that I
might write to him myself to ask him to examine it. UnlessI’m much mistaken, he’ll be kind enough to approve it. Then
you could also show it to a few others, as you judge fit. Onceapproved by three or four such people, it could be printed;
and if you agree, I would dedicate it to all the gentlemenof the Sorbonne, asking them to be my protectors in God’scause. For I must confess that Bourdin’s quibbles havemade me decide to do what I can to fortify myself henceforthwith the authority of others, since truth by itself is so little
esteemed.

1638–1640

most commonly used and whether there have been any newones in the past 20 years. . . . Also, I would like to know
if there is in current use any conspectus of the whole ofscholastic philosophy; this would save me the time it would
take to read their huge volumes. . . . And, finally, if you thinkit’s a good idea for me to dedicate my treatise on metaphysics
to the Sorbonne, please tell me what heading I should use
for my open letter to them at the start of the book.
[Five pages of physics and mathematics, and then:] I
entirely agree with the argument that you were sent byFather Lacombe of Blaye:(1) Whatever we conceive distinctly to be possible ispossible;(2) We conceive distinctly that it is possible that the world
was made;
therefore(3) The world was made.
(4) It’s certainly impossible to conceive distinctly that the sunor any other finite thing doesn’t depend on anything, because
independence—conceived distinctly—involves infinity. Nor
can we conceive distinctly that any atom or other portion ofmatter can occupy a larger or smaller space. First of all, an
atom can’t be conceived distinctly because the very meaningof ‘atom’ involves a contradiction—namely the contradictory
attributes of •being a body and •being indivisible. And any
distinct thought one can have of any portion of matter necessarily involves the thought of the determinate quantity of thespace occupied by it. The principal aim of my metaphysics
is to show what things can be distinctly conceived.
[Then a paragraph each on the tides and on light.]

I shan’t travel this winter, because I’m due to receive theobjections of the Jesuits some time in the next four or fivemonths, and I think I should hold myself in readiness forthem. And while I’m waiting for that I want to reread a littleof their philosophy for the first time in 20 years, to see if I’ll
think better of it now than I did before. For this purpose,
please send me the names of the authors who have written
textbooks of philosophy, and tell me which of them are theL[5.x.40: Descartes writes to Wilhelm asking for guidance on whatto advise Waessenaer in the matter of his dispute with Stampioen (see17.viii.40 letter to Wilhelm).]

112

CorrespondenceL

René Descartes

1638–1640

a body can’t be composed of indivisible things, because anindivisible thing can’t have any length or breadth or depth.
If it did, we could divide it at least in our imagination, andthat would show that it wasn’t indivisible: for if we could
divide it in imagination, an angel could divide it in reality.
He thinks motion and shape by themselves are inadequate
as principles [see Glossary] of explanation, because he doesn’t
see how all the properties of wine, for example, could be
explained in terms of them. You can remove this difficultyby telling him that they have all been explained already, as
have all the other properties perceptible by the senses. But
not a word about miracles. . . .

[7.x.40: Regius writes to Descartes, reporting on his recent publicdefence of his ·Cartesian· ideas against opponents in his university. We
don’t have this letter; only reports on it. It seems that Regius was thought
to have done extremely well, except that the dishonesty of his opponents
angered him, causing him to •forget the example he should be settingand •throw inappropriate jokes and sarcasm into the debate.]

to Mersenne, 28.x.1640:[This letter opens with two pages relating to Bourdin and hisallies—how they should be handled if they behave thus or
so. Then brief mentions of four bits of writing that Descartes
has received from Huygens: someone writing on whirlpools,
Debeaune against Desargues, Fermat on tangents, someone on ‘the earth’s daily movement’. Then three pages on
Mersenne’s latest questions and comments on the physics ofcollisions, things that float in water, tides, someone’s thesisthat material things are all made of salt. Then Descartes
turns to ‘the letter from one of your priests at Blaye’ [actually
Lacombe]. Four episodes in this are worth quoting here:](a) I don’t accept his indivisible bodies, or the natural
inclinations that he attributes to them. I can’t make sense ofsuch ‘inclinations’ except in things that have understanding;and I don’t attribute them even to animals that lack reason.
Everything in them that we call natural appetites or inclinations is explained on my theory solely through the rules ofmechanics. I can’t accept his ‘elements’ either; they are atleast as hard to understand as the things he tries to explainby them.
(b) If a •thing is made up out of two indivisible things,
then however you go about smashing it you’ll get only twoparts. But before saying that a •body could be made of two
indivisible things, you have to know what ‘body’ means. In
fact it means ‘thing that has length, breadth and depth’; so(c) I don’t see why he associates •atheism with •·thedoctrine of· those who explain nature in terms of shapes
and motions—as if the two were somehow alike or related.
(d) He says:‘The idea of a simple being, which we conceive tocontain all being, couldn’t be conceived if there weren’ta real exemplar of this being, because we can conceive[you should add “distinctly”] only things that are
possible and true.’
This makes it look as if he has read my works, which containthis very argument; but he adds many things that I cannot
agree with, such as that
this being has dimensions, and dimensions can beconceived without the thing that has the dimensions
being divisible,
and so on. He is right in saying that if we don’t conceivex distinctly it doesn’t follow that x is false. He does well toapply this to the mystery of the Trinity, which is an article offaith and can’t be known by natural reason alone. . . .
113

Correspondence

René Descartes

to Mersenne, 11.xi.1640:

1638–1640

philosophy. I won’t waste words, but will simply put down all
my conclusions with the true premises from which I derivethem. I think I can do this without many words. In thesame volume I plan to have printed a textbook of traditional
philosophy, perhaps Father Eustache’s, with notes by meat the end of each proposition. In the notes I will add thedifferent opinions of others, and what one should thinkof them all, and perhaps at the end I’ll compare the two
philosophies. But please don’t tell anyone yet of this plan,
especially before my Metaphysics is published; because if
the Regents knew of it they might do their best to steer mein other directions—whereas once the thing is done I thinkthey’ll all be pleased. Going public with my plan might also
block the Sorbonne’s approval ·for my Metaphysics·, which I
want, and which I think may be very useful for my purposes;
for the little book on metaphysics that I sent you contains
all the principles of my physics.

Thank you for your news of Voetius. I find nothing strange
in it except his not knowing that I am your friend; everyone
here who knows me at all knows about that. He is themost openly and completely pedantic fellow in the world,and he’s bursting with rage because there is a professorof medicine [namely Regius] in their University of Utrecht
who openly teaches my philosophy, and even gives private
lectures in physics which in a few months equip his pupils
to make fun of the old ·scholastic· philosophy as a whole.
Voetius and the other professors have done their best to get
the magistrates to forbid him to teach, but the magistrates
allow him to continue. This Voetius has also ruined Mllede Schurmans: she had excellent gifts for poetry, painting
and other fine arts, but these last five or six years he hastaken her over so completely that all she cares about aretheological controversies, so that no decent people want totalk to her. . . .
I don’t think that the differences of opinion among thescholastics makes their philosophy hard to refute. It’s easy
to overturn the foundations on which they all agree, andthen all their disagreements over detail will seem foolish. I
have bought the Philosophy of Father Eustache of St Paul,
which seems to me the best book of its kind ever made. I
would be glad to know if the author is still alive. [He was, but
only for another month.]. . . .
I would willingly answer your question about the flame ofa candle and similar things; but I see that I can’t ever reallysatisfy you on this until you have seen all the principles ofmy philosophy; and I tell you now that I’m determined towrite them up before I leave this country, and to publish
them perhaps within a year. My plan is to write a seriesof theses that will constitute a complete textbook of my[Several paragraphs about recent works by various writers; about Cardinal Bagné (he still remembers Descartes,
who thinks he should be sent a copy of the Metaphysics
when it is printed); about troubles with the postal service(some Descartes’s mail arrives already opened, he thinks by‘the messenger’).]Yesterday I sent my Metaphysics to Huygens, to post
on to you; but he’ll delay that for a week, which I haveallowed him to look at it. I haven’t put any title on it,
but it seems to me that the most suitable would be René
Descartes’s Meditations on First Philosophy [he gives it in
Latin], because I don’t confine my discussion to God andthe soul, but deal in general with all the first things to bediscovered by philosophising. . . .
114

Correspondence

René Descartes

to Gibieuf, 11.xi.1640:

1638–1640

to Mersenne, 11.xi.1640:
At last I am sending you my work on metaphysics, which I
haven’t yet given a title to, so as to make you its godfather
and leave the baptism to you. As I said in my most recent
letter to you, I think it could be called Meditationes de Prima
Philosophia, because in it I deal not just with God and thesoul but in general with all the first things that can be
discovered by philosophising in an orderly way. And myname is so widely known that if I didn’t put it under thetitle I would be thought to be engaging in something tricky,
motivated by vanity rather than modesty.
As for the letter to the gentlemen of the Sorbonne, if my
opening form of address is inadequate, or I have left out
some closing salutation or other ceremony, please insert
it; I don’t think it will lose anything by not being in myhand-writing. I am sending you the letter to the Sorbonne
under separate cover from the treatise itself, because I thinkthat if all goes well the best plan would be this: once all
the material has been seen by Gibieuf and (please!) by one
or two of your friends, let the treatise be printed minus theletter, because the letter is stylistically so bad that I don’t
want many people to see it, and let the printed version thenbe presented to the Faculty of the Sorbonne, together withthe letter in manuscript. [Descartes’s reason for ‘minus the letter’

The honour you did me, several years ago, of telling methat you didn’t find my philosophical views incredible, andmy knowledge of your exceptional learning, give me a strong
desire that you would kindly look at the work on metaphysics
that I have asked Mersenne to send you. The route that I taketo •show the nature of the human soul and •demonstrate
the existence of God is, I believe, the only one that can takeus there. No doubt others could have made better use ofthis path than I have, and I’ll have left out ·explanationsof· many things that needed to be explained; but I’m surethat I can make good all the defects, provided I am alerted
to them, and that I can make my proofs so evident and so
certain that they can be taken as demonstrations. But one·potential· defect remains: I can’t ensure that people of every
level of intelligence will be able to understand the proofs, oreven that they’ll take the trouble to read them attentively
unless they (the proofs) are recommended by people otherthan myself. I know of no people on earth who can give sucha recommendation more effectively than the gentlemen of
the Sorbonne, or anyone that I would look to for a moresincere appraisal; so I have decided to seek their special
protection. And because you are one of the leading lights of
the Sorbonne Society, and have always done me the honourof giving me signs of your affection, and above all because
it is the cause of God that I am defending, I look to you forhelp in this matter. I rely on you to advise Mersenne on
how he should conduct this business, and on your kind help
in securing favourable judges for me, and in being one ofthem. In so doing, Reverend Father, you will oblige me to be
most devotedly for the rest of my life your very humble andobedient servant, Descartes.

is à cause que la copie en est trop mal écrite pour être lue de plusieurs,
which more naturally means ‘because the handwriting of the manuscript
is so clumsy that not many people could read it’; but that doesn’t makesense as a reason for not printing the letter along with the treatise, so theless natural reading has been preferred.]

The fairest way of proceeding after that would, I think, be
for the Faculty to delegate some of their number to examineit; so we’ll need to provide them with as many copies as
they need for this purpose—or rather with as many copies
115

Correspondence

René Descartes

as there are doctors [see Glossary] in the Faculty. If they findanything to object to, they should send me their comments
for my reply, and this could all be printed at the end of thebook. After that, I don’t think they could refuse to give theirverdict on the book, which could be printed at the beginning
together with my letter to them. But things may turn out
quite differently from what I expect, which is why I put myself
entirely in your hands and Gibieuf’s (I’m asking him by letterto help you conduct this business). The recent skirmish
against me of which you are aware has made me realise that
however just one’s cause may be one still needs friends to
defend it. . . .
L(1) we exist,(2) we know that we exist, and(3) we love the existence and the knowledge we have;whereas I use the inference to show that this I that is
thinking is an immaterial substance with no bodily element.
These are two very different things. To infer that one exists
from the fact that one doubts something is such a simple andnatural thing that it could have occurred to anyone. Still,
I’m very glad to find myself in agreement with St Augustine,
if only to hush the little minds who have tried to find fault
with this principle. My little book on metaphysics is alreadyon the way to Paris, where I think it will be printed; all that
I have left is a draft so full of crossings out that I could
scarcely read it myself, which is why I can’t let you have it.
But as soon as it is printed, I will see that you receive a copy
as soon as anyone, since you are kind enough to want toread it, and I’ll be glad to have your opinion of it.

[12.xi.40: Descartes writes to Huygens, asking for his judgmenton the Meditations. He says (in effect) that properly judging the work
would take ‘whole days and weeks of meditation’, but, aware of the otherdemands on Huygens’s time, he only asks him to read through, at asitting, the first five Meditations and Descartes’s response to a letter thatwill be published with the Meditations.]

L

1638–1640[18.xi.40: Descartes writes to Mersenne: about how to get things

to Mersenne, 3.xii.1640:

forwarded to Huygens; Desargue’s defence of Descartes against Bourdin;
the propriety of Mersenne’s forwarding to Descartes Bourdin’s latest;

What you report from St Augustine and St Ambrose—that ourheart and our thoughts are not in our power. . . ,—applies
only to the sensitive part of the soul, which receives theimpressions of external or internal objects. . . . I entirely
agree with them about that; I have never said that •all ourthoughts are in our power but only that •if there is anything
absolutely in our power, it is our thoughts, namely the ones
that come from our will and free choice. There’s no conflict
here between them (·those two saints·) and me; all I wanted
in writing that was to get across the point that our free will
has no absolute jurisdiction over any corporeal thing, ·sothat if it has such jurisdiction over anything it must be overthoughts·. This is true and undeniable.

remarks about Mersenne’s response to some theological objections to
Descartes; physics of projectiles etc.]

to Colvius, xi.1640:
I am obliged to you for drawing my attention to the passageof St Augustine [The City of God XI:26] relevant to my ·inference·
I am thinking, therefore I exist. I went today to the Leiden
library to read it, and I find that he does indeed use it toprove the certainty of our existence, and goes on to showthat there’s a certain likeness of the Trinity in us because·of the triple-fact that·116

Correspondence

René Descartes[Two pages of miscellaneous material—Bourdin, Fermat,
Desargues, physics, a suggested alternative to Eustache’sbook as a brief and accessible introduction to Scholastic
philosophy. Then:] Your most recent letter tells me of thedeath of my father [Joachim Descartes, died 17.x.40]. This gives
me great sadness; and I greatly regret not having been able
to go to France this summer, to see him before he died. But
since God didn’t allow this, I expect to stay here until my
Philosophy [here = Principles of Philosophy] is completed.

1638–1640

my principles, are trying to persuade the world that I havesome radically false opinions. (iii) I would be glad if hewould sound out the Cardinal on this subject, because as
his obedient servant I would be very sorry to displease him,and because as an earnest Catholic I have a general respect
for all the Catholic Church’s leaders. I don’t add that I’m
reluctant to risk their censure; I have firm faith in the
Church’s infallibility, and have no doubts about my own
arguments—I can’t be afraid that one truth may conflict withanother!
You are right to say that we are as sure of our free will asof any other primary notion; for this is certainly one of them.
When one candle lights another [Mersenne had asked aboutthis], this is merely a single fire spreading from one wick toanother. The particles of the flame are agitated by very subtle
matter, and so have the force to agitate and separate theparts of the second wick. The fire thus grows, and then is
divided into two fires when the two wicks are separated.
But I can’t give a good account of fire without presenting
the whole of my philosophy, and I tell you in confidence that
I’m starting to make a summary of it. I propose to lay out
the entire course in proper order, so as to have it printed
along with a compendium of scholastic philosophy (like theone Eustache made). At the end of each Question [roughly =‘each chapter’] I will append my own notes in which I’ll report
the opinions of the various authors and say what one should
think of them all and how useful they are. I think I can dothis in a way that will make it easy to see how scholastic
philosophy compares with mine; and those who haven’t yet
learned scholastic philosophy will learn it more easily fromthis book than from their teachers, because they will learn
to scorn it at the same time. As for my own philosophy, even
the weakest teachers will be able to teach it from this bookalone. If Father Eustache is still alive, I won’t use his bookto Mersenne, xii.1640:[A response of Descartes’s to Bourdin’s attack may have been
held up by Mersenne—Descartes conjectures—because hethought it would give offence and make it hard for the two
ever to become friends. Descartes tells Mersenne to forward
the response, which may do some good, because:] when hesees that I have a beak and talons to defend myself, he may
be more restrained in what he says about me from now on.
[More than two pages on music, the physics of subtle
matter, the nature of gold, why there are no tides in lakes,
and a tangle of oppositions and cross-purposes involving thelibrarian Heinsius [see Huygens’s letter of 30.vii.38] and several
other people. Then:] I’m not sorry that the ministers arethundering against the movement of the earth; perhaps thiswill encourage our own preachers to give it their approval! A
propos of that, if you are writing to Cardinal de Baigné’s
physician ·Gabriel Naudé·, please tell him ·these three
things·. (i) The only thing that has stopped me from publishing my philosophy up to now is the matter of defending
the movement of the earth; I couldn’t separate this from myphilosophy, because the whole of my physics depends on it.
(ii) I may soon be forced to publish my philosophy, becauseof the slander of people who, having failed to understand117

Correspondence

René Descartes

1638–1640

could not annihilate the soul, but only that •it is by natureentirely distinct from the body, and therefore not boundby nature to die with it. This is all that’s required as afoundation for religion, and all that I intended to prove.
You shouldn’t find it strange, either, that I don’t prove in
my Second Meditation that the soul is really distinct fromthe body, but merely show how to conceive it without the
body. At that point in the work I don’t yet have the premises
needed for that conclusion, but the conclusion does show
up in the Sixth Meditation.
It should be noted that in this work I don’t follow theorder of the •subject-matter but the order of the •reasoning.
I don’t try to say in one place everything relevant to a given
subject, because some of it could be defended only withreasons that aren’t available until later in the work. Instead,I reason in an orderly way from what is easier to what isharder, making what deductions I can, now on one subject,
now on another—this being the right way (in my opinion) tofind and explain the truth. The order of the subject-matteris good only for those whose reasoning is disjointed,

without permission; but it’s not yet time to request it, or evento mention this plan; I need first to see how my meditations
on metaphysics are received.
[Two paragraphs about •reflection and refraction and•Bourdin.]
I shall look at St Anselm at the first opportunity. Sometime ago, you drew my attention to a passage from St
Augustine concerning my I am thinking therefore I exist, andI think you have asked me about it again since then. It is in
The City of God XI:26.

to Mersenne, 24.xii.1640:
The difficulty you raise about the pineal gland seems to be
the most urgent, and the man [Dr Villiers of Sens] who wants
to defend publicly what I said about it in my Optics doesme so much honour that I must try to answer his queries.
So without waiting for the next post I will say. . . [and thentwo pages of mostly anatomical description whose details
we needn’t follow.] [An oddity in this passage should be noted.
When in the present version Descartes speaks of ‘the pineal gland’ what

the rest of the sentence: et qui peuvent dire autant d’une

he actually wrote was ‘the conarium’, which all scholars agree was his
name for the pineal gland. But in the passage now being omitted hedifficulté que d’une autre.

speaks both of ‘the conarium’ and (just once) of ‘the glandula pineala’;literally meaning: and who can say as much about one

the passage as a whole suggests that these are meant to be two names

difficulty as about another.

for one thing, but Descartes does not outright say so.]perhaps what Descartes is getting at: and who are willing

I am greatly indebted to you for the care you are taking
over my book of metaphysics, and I give you a free hand tocorrect or change whatever you think fit. But I’m astonishedthat you promise me the objections of various theologians
within a week, because I was sure it would take longer thanthat just to read it carefully. . . .
You shouldn’t be surprised that I haven’t said a word
about the immortality of the soul. I couldn’t prove that •God

to tackle any question as it comes up, with no concern forwhether this is the best place to tackle it.
So I don’t think it would be useful or even possible to insert
into my Meditations the answers to the objections that may
be made to them. That would interrupt the flow and evendestroy the force of my arguments. Most objections would
be drawn from things that are perceivable by the senses,
118

Correspondence

René Descartes

1638–1640

whereas my arguments depend for their force on readers’willingness to withdraw their thought from these things. . . .
I hope that people will take their time in composing theirobjections; it doesn’t matter if the treatise remains unpublished for two or three more years. The manuscript—·justbecause it is a manuscript·—can be seen by only one person
at a time, and is very ill-written; so I think it would be useful
to have twenty or thirty copies printed in advance. I’ll happily
pay whatever it costs; I would have had it done here if there
were any publisher that I could trust; but I did not want theministers [here = ‘government officials’] of this country to see it
before our theologians.
[A paragraph about style, and imperfections in the Latin.]
I will send you perhaps within a week an abstract of theprincipal points concerning God and the soul, which canbe printed in front of the Meditations so that people can see
where such matters are to be found. Otherwise many people
will be annoyed at not finding in one place everything they
are looking for. I shall be very glad to have Desargues added
to my roster of judges, if he is willing to take the trouble;
I have more trust in him than in ·any· three theologians. I
won’t be at all unhappy to have many objections, because I’msure they will serve to make the truth better known. Thank
God, I have no fear of being unable to reply adequately. It istime to finish.

Several people here have told me that a number of Jesuit
priests are speaking against my writings, and this has led afriend of mine to write a treatise in which he plans to makea full comparison between the philosophy that is taught inyour schools and the philosophy that I have published. He
aims, by showing what he thinks bad in one philosophy, tomake it easier to see what he thinks is better in the other.
I thought that I shouldn’t go along with this plan until I
had told you about it, and asked you to tell me what you
think I should do about it. •My obligations to your priests
for my education in my youth, •the strong inclination I havealways had to honour them, and •my preference for gentle
and friendly procedures as against ones that might upset
people—these would all be strong enough reasons for me
to ask my friend to choose some other topic to write about,a topic that doesn’t involve me, if I weren’t virtually forced
to go the other way by •what I’m told about the harm it
would do me if I stayed silent and by •the rule of prudence
that it’s much better to have declared enemies than covert
ones. Especially in a matter like this, where. . . .the louder
the battle is the more advantageous it will be to the one
who is in the right. But the respect that I owe you, and theaffection that you have always kindly showed towards me,
have more force for me than anything else and cause me towait to hear your commandments on this subject.

to Charlet, xii.40:

to Mersenne, 31.xii.1640:

I know that you are very occupied with work that mattersmore than reading letters from someone who isn’t in aposition to do anything for you; I hesitate to confront you
with a request to do something for me, though I have noother reason for writing to you except to assure you of mygreat respect for you.

Responding to points of yours that I didn’t have time to coverin my letter a week ago: First, I send you an abstract of my
Metaphysics, which, if you approve, can be prefaced to thesix Meditations. . . . The reader will be able to see in it a shortstatement of everything I have proved about the immortalityof the soul, and everything that I can add to that when I
119

Correspondence

René Descartes

1638–1640

it follows from the soul’s •being distinct from the body and•having thinking as its essence.
You find obscure the sentence ‘Whatever has the power
to create or preserve something separate from itself has
a fortiori the power to preserve itself’. I don’t see how tomake it clearer without adding many words, which would be
stylistically bad because I mention the matter only briefly by
the way. . . .
It seems very clear to me that possible existence is contained in everything that we clearly understand, becausefrom the fact that we clearly understand something it follows
that it can be created by God.
As for the mystery of the Trinity, I share St Thomas’s
opinion that it is a sheer article of faith and can’t be known
by the natural light [see Glossary]. But I do not deny that there
are things in God that we don’t understand, just as evena triangle has many properties that no mathematician will
ever know—and yet everyone knows what a triangle is.
It is certain that there is nothing in an effect that is notcontained formally or eminently [see Glossary] in its efficientand total cause. I added ‘efficient and total’ on purpose. The
sun and the rain are not the total cause of the animals theygenerate.
I was finishing this when I received your last letter, whichreminds me to ask if you know •why you didn’t receive my
Metaphysics by the post by which I sent it, or even with theletters I wrote a week later, and •whether the packet was
opened; for I gave it to the same messenger.
[He thanks Mersenne for correcting a solecism in his
Latin, which he notes that several of his friends didn’t notice.
Then:] I have no objection to seeing what Morin has written
about God, because you say he uses a mathematical method;
though (between ourselves) I don’t expect much from it,
because I never heard before that he went in for that sortpublish my Physics. Without wrecking the order I could notprove that the soul is distinct from the body before proving
the existence of God.
You say that ‘we don’t know that the •idea of a mostperfect being isn’t the same as that of •the corporeal world’;
but in fact it is easy to know this, in the same way that wecan proved that the soul is distinct from the body, namelyfrom the fact that we conceive something altogether different
in each case. But this works as a proof only if we form clear
ideas of the things we want to judge about, and ordinary folk
don’t do that; ·the importance of doing· this is what I havemainly tried to teach by my Meditations. But I won’t spend
longer on these objections, because you promise to send me
shortly all the objections that can be made. But I only ask
that nobody be in a hurry about this: people who don’t study
everything carefully—who merely read the Second Meditation
to see what I say about the soul, or the Third to see what
I say about God—will very likely raise objections against
things that I have already explained.
In the place where I put ‘in accordance with the laws of
my logic’ please put ‘in accordance with the laws of the true
logic’; it’s near the middle of my Replies to Caterus, where heobjects that I have borrowed my argument from St Thomas.
The reason why I add ‘my’ or ‘the true’ to ‘logic’ is that I haveread theologians who follow the ordinary logic and inquire
what God is before inquiring whether God exists. . . .
[Here and below, ellipses. . . .replace short passages aboutother suggested changes in Descartes’s text, sometimes
accompanied by sharp declarations that what he originallywrote is not obscure—‘thousands of things in Cicero are moreso’, the point being that Cicero has always been regarded asa model of clarity.]
As for my saying ‘Nothing can be in me, i.e. in my mind,of which I am not aware’, I proved this in my Meditations;120

Correspondence

René Descartes

of writing. . . . Huygens has returned, and if you send it tohim with the paper by the Englishman [Hobbes] I can getthem from him. But do ask him to send them on promptly,
because he has so much other business that he might forget.
I won’t fail to answer immediately anything you send me
about my Metaphysics. But apart from that I would be glad
to have as few distractions as possible, at least for the coming
year, which I have resolved to spend writing my philosophy

1638–1640

in an order that will make it easy to teach [namely the Principlesof Philosophy]. The first part, which I am working on at present,
contains almost the same things as the Meditations that you
have, except for being in an entirely different style—and whatis written at length in one is abbreviated in the other, andvice versa.
[The letter ends with a page on various personal matters.]

121