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would be more forceful if Aquinas could say that the mover must
be actually F, but he cannot say that, at least not with perfect generality.
For Aquinas thinks that God can move things in many ways that God is
not actually: God can fatten a man without himself being fat. In that case,
God is said to be virtually F, where something is β€œ virtually F ” if it is not
itself F but it has the power to make others F. One may say, then, that
something is in a state of actuality relevant to F when it is either actually
F or virtually F.
It is certain, and obvious to the senses, that in this world some things are
moved. But everything that is moved is moved by another. For nothing is
moved except insofar as it is in potentiality with respect to that actuality
toward which it is moved, whereas something effects motion insofar as it is
in actuality in a relevant respect. After all, to effect motion is just to lead
something from potentiality into actuality. But a thing cannot be led from
potentiality into actuality except through some being that is in actuality in a
relevant respect; for example, something that is hot in actuality – say, a fi re
– makes a piece of wood, which is hot in potentiality, to be hot in actuality,
and it thereby moves and alters the piece of wood. But it is impossible for
something to be simultaneously in potentiality and in actuality with respect
to same thing; rather, it can be in potentiality and in actuality only with
respect to different things. For what is hot in actuality cannot simultaneously
be hot in potentiality; rather, it is cold in potentiality. Therefore, it is impossible
that something should be both mover and moved in the same way and
with respect to the same thing, or, in other words, that something should
move itself. Therefore, everything that is moved must be moved by another.
If, then, that by which something is moved is itself moved, then it, too,
must be moved by another, and that other by still another. But this does not
go on to infi nity. For if it did, then there would not be any fi rst mover and,
as a result, none of the others would effect motion, either. For secondary
movers effect motion only because they are being moved by a fi rst mover, just
as a stick does not effect motion except because it is being moved by a hand.
Therefore, one has to arrive at some fi rst mover that is not being moved by
anything. And this is what everyone takes to be God. (ST I, q2, a3, response)
12 Timothy J. Pawl
P1. Some things are moved.
P2. If something is moved to being F, then it is potentially but not actually
F.
P3. If something moves a thing to be F, then it (the mover) is in a state of
actuality relevant to F.
C1. If something were to move itself to be F (e.g., be both moved and
its own mover), then it would be both potentially but not actually F
and also in a state of actuality relevant to F (conjunction, and modus
ponens , P1, P2, P3).
P4. But it is not possible for something to be both potentially but not actually
F and also in a state of actuality relevant to F.
C2. It is not possible for something to move itself to be F ( modus tollens,
C1, P4).
P5. If it is not possible for something to move itself to be F, then if something
is moved, it is moved by something else.
C3. If something is moved, it is moved by something else ( modus ponens ,
C2, P5).
P5. If B moves A and B is moved, then B must be moved by some other
thing, C. And if C is moved, then C must be moved by still some other
thing, D. And so on.
P6. If the series of movers were to go on to infi nity, then there would be
no fi rst mover.
P7. If there were no fi rst mover, then there would be no motion.
C4. There is a fi rst mover ( modus tollens , P1, P7).
C5. That fi rst mover is the thing that everyone takes to be God
(defi nition).
The Second Way – The Argument from Causation
Whereas the First Way focused on accidental changes, the Second Way
focuses on ordered series of effi cient causation. An effi cient cause is that
which produces something or an alteration in something. The composer is
the effi cient cause of the sonata; the fi re is the effi cient cause of the heating
of the kettle. An ordered series is a series in which the causal work of later
members in the series depends on the simultaneous causal work of earlier
members in the series. If the fi re heats the kettle and the kettle heats the
water, it is an ordered series, since the kettle ’ s heating the water depends
upon the causal activity of the earlier cause, the fi re. Likewise, a system of
gears is an ordered causal series, since the causal action of one intermediate
gear spinning another, later gear depends upon the causal activity of previous
gears in the system. Aquinas argues in the Second Way, to continue
with the gear image, that the system cannot be gears all the way back. An
Aquinas’ Five Ways 13
infi nite series of gears, without a fi rst cause of their spinning, would not be
in motion.
We fi nd that among sensible things there is an ordering of effi cient causes,
and yet we do not fi nd – nor is it possible to fi nd – anything that is an effi cient
cause of its own self. For if something were an effi cient cause of itself, then
it would be prior to itself – which is impossible.
But it is impossible to go on to infi nity among effi cient causes. For in every
case of ordered effi cient causes, the fi rst is a cause of the intermediate and the
intermediate is a cause of the last – and this regardless of whether the intermediate
is constituted by many causes or by just one. But when a cause is
removed, its effect is removed. Therefore, if there were no fi rst among the
effi cient causes, then neither would there be a last or an intermediate. But if
the effi cient causes went on to infi nity, there would not be a fi rst effi cient
cause, and so there would not be a last effect or any intermediate effi cient
causes, either – which is obviously false. Therefore, one must posit some fi rst
effi cient cause – which everyone calls God. (ST I, q2, a3, response)
P1. There is an ordered series of effi cient causes.
P2. Necessarily, if X is an effi cient cause of Y, then X is prior to Y.
C1. Necessarily, if X is an effi cient cause of X, then X is prior to X
(instantiation, P2).
P3. It is not possible for X to be prior to X.
C2. It is not possible for X to be an effi cient cause of itself ( modus tollens ,
C1, P3).
P4. If something is an ordered series of effi cient causes, then the fi rst cause
causes the intermediate cause(s), and the intermediate cause(s) cause(s)