188 ARGUMENTS: MONOTHEISTIC CONCEPTIONSDefinition 8: Y causes X to change with respect to property Q if and
only if Y causes X to move from potentiality to actuality,
or from actuality to potentiality, with respect to Q.
The First Way
The First Way concerns things being caused to move, in the usual sense of
going from one place to another but also in the more general sense of
simply changing, in position or some other way. It is assumed that X
changing Y with respect to Q is a relationship that is non-reciprocal – is
analogous to non-reciprocal dependence. Thus the idea is that it cannot be
the case that X’s changing Y with respect to Q causes Y’s becoming Q only
if Y’s becoming Q causes Y’s causing Z to become Q and Z’s becoming Q
causes X’s causing Y to become Q. The argument goes as follows.
1 Some things change.
2 If X changes at time T then there is something Y that changes X at T
(understood as that Y is something different from X).
3 Either (a) there is an infinite series of changed and changing beings
(i.e., a series each member of which is both a changed and a changing
thing) or (b) there is some being that is a changing being (a cause of
motion/change) but is not a changed being (something that changes).
4 Not-(a). So:
5 (b).
Aquinas offers a subsidiary argument for premise 2:
2a For all times T, and all X, X is in actuality with respect to moving at
T or X is in potentiality regarding motion at T.
2b If X is in potentiality regarding motion at T, then X is not moving
atT.
2c If X is not moving at T, then X cannot cause motion at T.
2d Nothing can cause its own motion at T (since in order to do so it
would have to both be in actuality regarding motion at T and in
potentiality regarding motion at T, and that is impossible).
2e No motion can be uncaused.
Hence:
2 If X changes at time T then there is something Y that changes X at T
(understood as that that Y is something different from X).