# On the Vital Principle/Book 2/Chapter 7

The visible is that for which vision is the sense, and the visible is both colour and something which is describable by words, although it happens to be without a name; but our meaning will become clear to those who accompany us in the inquiry. The visible is colour, and colour is that which is upon something visible in itself; and this something is visible, not only after its appellation but, because it has in itself the cause of being visible. All colour is motive of the diaphanous, in activity, and to be so motive is the nature of colour. On which account nothing is visible without light, but the colour of each object is visible in the light; and we must, therefore, first say what light is. There is a something diaphanous, and I call diaphanous what is visible, and yet not visible, ​strictly speaking, in itself, but made visible by colour, which is foreign to it. Such is air, and water, and many solid bodies; yet neither air nor water, as air or water, is diaphanous, but the same nature is present in both those elements, which is in the eternal supernal body. Light is the active state of that same diaphanous, in so far as it is diaphanous, and darkness is the same in its state of potentiality. But light is the colour, as it were, of the diaphanous, when made diaphanous in reality by fire, or other such element as the supernal body; for to it belongs a something which is identical with fire. We have thus said what is the diaphanous and what light, and have shewn that neither of them is fire, nor a body, strictly speaking, nor an emanation from a body, (as, in that case, they would be corporeal), but that they are the presence in the diaphanous of fire or something analogous to fire, since two bodies cannot possibly coexist in one and the same body.
Light seems to be the opposite to darkness; and as darkness is the absence of a particular state of the diaphanous, it is evident that the presence of that state must be light.
Thus Empedocles, or whoever else may have held the same opinion, was wrong in supposing that light was transported and manifested, without our consciousness, between the Earth and surrounding space; for the opinion is opposed as well to sound conclusion ​as to observation of the phenomenon. If the interval were small, the fact might, indeed, escape us; but, extended as it is from the East to the West, the postulate is too extravagant to be admitted.
Now that which is without colour is receptive of colour, as that which is without sound is receptive of sound; and that which is without colour is the diaphanous and the invisible or scarcely visible, such as darkness seems to be. Such too is the diaphanous; but then it is the diaphanous, not in potentiality but, in reality; for the same nature is sometimes darkness and sometimes light. But all objects are not visible in light, as there are some of which the peculiar colour only of each is visible; for some, not visible in light, produce sensation in the dark, as certain fiery brilliant appearances (which have no special appellation,) which emanate from fungi, horn, scales and eyes of fishes, but the peculiar colour is not seen of any one of those objects. It is foreign to our present purpose to explain how such objects become visible; but this much is manifest, that it is colour which is visible in light. Therefore, without light colour is not visible; for it is an essential property of colour to be motive of the diaphanous in activity, and the reality of the diaphanous is light. As proof of this, if any coloured object be placed over the sight, the object will not be seen, and yet there is colour, which is motive of the diaphanous, the air, that is, and, by ​its being continuous between the object and the sense, it is able to give motion to the visual organ. Thus, Democritus was wrong in thinking that if the medium were a void, vision would be so accurate as to render an ant visible in the sky. The opinion, in fact, involves an impossibility; for vision is produced by some kind of impression upon the visual organ, and as this cannot possibly be effected by the colour which is visible, there remains only that it must be by the medium, and thus a medium there must be; so that if there were a void, vision would be, not to say inaccurate but, altogether precluded.
It has thus then been said why colour must be visible in the light; but fire is visible both in darkness and in light, and necessarily so, since it is by fire that the diaphanous becomes diaphanous. The same reasoning holds good for sound and for odour, as nothing sonorous or odorous can produce sensation when in immediate contact with the sentient organ; but by odour or sound the medium is set in motion, and by it the organ is moved. Thus, when any thing sonorous or odorous is placed immediately upon the sentient organ, no sensation is given out; and this is the case with the sense of Touch, although less evidently so; but the cause of this shall be explained hereafter. The air is the medium for sounds, while that for odour has no special appellation, for there is a particular impression common to air and water; and ​what the diaphanous is to colour that which is in those elements is to odorous bodies, as aquatic animals appear to be sensible of odours. But neither man nor animals which breathe can smell without inspiring; and the cause of this shall be spoken of hereafter.

## Notes

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​Note 1, p. 93. The visible is colour, &c] Aristotle says that the faculty of Sight announces to us, distinguishes, that is, the manifold and various shades of colours, on account of all bodies partaking of colour, and thus by Sight, especially, we are able to perceive common properties, such as form, magnitude, motion and number; but the Hearing, on the contrary, is perceptive only of distinctions of sounds from sonorous bodies and the variations of voice from such as have speech. The sense of "Hearing, however, contributes more than any other, since speech is the channel for instruction, to the cultivation of the understanding."
Note 2, p. 93. All colour is motive of the diaphanous, &c.] These passages seem almost to indicate a presentiment of the modern or undulatory theory of light, for they assume the existence of a diaphanous, that is, a subtle medium which, by its motion, is creative of vision. So too, the modern theory assumes a subtle elastic ether, which has inertia without gravity, which fills space, permeates all bodies, and admits of being set in motion by the agitation of the particles of ponderable matter, and which particles, when set in motion, communicating a like ​motion to the molecules adjacent, act upon others, and thus motion is propagated further and further in all directions. The theory of Aristotle is much the same— there is a diaphanous medium which may well represent the subtle ether, and which, when potential, that is quiescent, is darkness, and when set in motion by colour, (the property of which is to render it motive), is light, renders objects visible, that is. Thus, the same diaphaneity when passive, that is, potential, is darkness, when active, that is, in reality, is light, and the cause of objects being visible. The value of the hypothesis is diminished by the identification of the "diaphaneity" with air and water and solid bodies, because of their affinity with the supernal region or firmament above, which, together with all the heavenly bodies, was supposed to be of igneous nature; and to be corporeal, circular, and in constant motion.
Note 3, p. 94. Light is the active state, &c.] The diaphaneity which, when passive, is darkness, when set in motion and made active, is light, is made visible, that is; and thus light, being a mere condition of the diaphaneity, "is not a body, for, were it so, there would be two bodies in one, which is an impossibility." It may now seem strange that Aristotle should have paid so little attention to the opinion of Empedocles, "that light arrives midway from the sun, before it reaches the sight, or the earth;" for although it differed from his own, in regarding the sun ​as the source of light and the distinction of day from night, yet, in transmitted light, it supplied a motor, which was required for the completion of his own theory of sensation through the agency of a medium acted upon by impulsion.
Note 4, p. 95. Now that which is without colour, &c.] The diaphaneity, that is, when passive, is receptive of colour and made active, just as the air, when quite still, is more readily set in motion and made sonorous by percussion; and this leads, amid some confusion of thought, to the consideration of those luminous appearances (ignes fatui) which are visible only in the dark, by their colour. 'The precise nature of these appearances is still only conjectural, notwithstanding the advance of chymistry; but they are supposed to be due to phosphyretted hydrogen eliminated, under favouring circumstances, from decaying animal and vegetable matter, and ignited by contact with the atmosphere."
Note 5, p. 95. Therefore, witliout light colour is not visible.] Colour, that is, by imparting motion to the diaphaneity, renders it, from being potential and dark, actual and visible, that is, light; and thus, as without light there is no colour, so without colour there is no light; and this lends support to the opinion, that the air, as being a diaphanous medium, is essential to sight. Aristotle had indeed maintained, in opposition to Empedocles and others, that vision is not caused by the ​emanation of luminous rays from the eye as light proceeds from a torch or lamp; and he ridiculed the notion that vision is precluded in the dark owing to the extinction of those rays therein. It is probable that this theory first led him to adopt a medium and its successive motion, as the immediate cause of vision; as he had accounted for hearing by the propagation of the impulse given to the air by the sonorous body. Aristotle was unacquainted with the structure of the eye; but he was aware, of course, that it contains humours, and these he held to be necessary, not as being aqueous that is elementary but, as being diaphanous, for this property seemed to be as requisite for vision within the eye, as it is for the transmission of light to the eye. It was this assumed succession of action, after impression upon a diaphanous medium, which led to the conclusion that the eye itself must be diaphanous, and, therefore, that the visual power must be somewhere on the inside of the eye; and this is the only approximation to a right knowledge of the retina and its relations.
Note 6, p. 96. It has thus then been said, &c.] The cause of colour being visible is sufficiently obvious from what has been said; but fire was said to be visible both in darkness and in light, owing to its being, as fire, of the nature of the firmament above, which was believed to be fire, or something identical with fire. It may be presumed that the subject was here introduced, in order to notice and account for those luminous appearances, ​which have been alluded to, and which, in that age, could not but have been topics of wonder and speculation ; they were irreconcilable besides, with the prevailing notions of colour and light.
Note 7, p. 96. The air is the medium for sounds, &c.] The air was by Aristotle held to be essential to sound; but it is not apparent why odour was supposed to be transmitted by some modified condition of air or water, unless, indeed, it was required in order to account for the perception of odours by fishes and aquatic animals. There was a difficulty, in fact, in accounting for the transmission of odour through air and water, because odour was held to be a vaporous exhalation eliminated by fire; and the "special organ of smell was said to be located about the brain," the coldest of all parts of the body, in order that the exhalation might there be condensed and made productive of smell. Thus, it might seem to be irreconcilable with odour, that it should be transmissible in air or water, and this may have led to the hypothesis of a modified condition of the elements for smell.
Note 8, p. 97. But neither man nor animals which breathe, &c.] The term in the text (ἀνάπνει), like our own term breathing, is expressive both of inspiration and expiration, whereas it is evident that the sense of the passage requires the former process only. And yet ​elsewhere, Aristotle, in his criticism of the theory adopted by Diogenes and Anaxagoras to account for the respiration of fishes, has clearly distinguished the one from the other. He objected also to Timæus and some others who had maintained that expiration must precede the other. Enough, however, that he perceived, although unacquainted with the parts on which odours impinge, or the organ by which they are made sensible, that they could gain access to the sense only through inspiration.
* ↑ De Sensu et Sens. I. 10.
* ↑ I. 11.
* ↑ Meteorologica, 3. 2.
* ↑ De Cœlo, 2. 3.
* ↑ De Sensu et Sens. II. 15, 16.
* ↑ De Sensu et Sens. II. 19, 20.
* ↑ De Part. Animalm, II. 7.
* ↑ De Respiratione, 2. 3.