While we’re on the topic of confusing things Aquinas said, we can talk about his analysis of change, which he in turn gets from Aristotle.

We’ve noted before that the first step in analysing change is the realisation that it involves the actualisation of a potential:

When a hot cup of coffee gets cold, for example, what is happening is that the cup’s potential for the being cold is actualised by the coldness in the surrounding air… When I pick the cup off the ground and place it on the desk, I am actualising the cup’s potential to be a meter above the ground

But, as we noted, not all such actualisation of a potential involves change. The thing that sets change apart from other actualisations of potentials is that it involves the movement from potential to actual. It is on account of this that the ancients and Scholastic happily used the words “motion” and “change” somewhat interchangeably.

Now, while calling change the movement from potential to actual serves as a helpful start it is by no means the end of a satisfactory analysis. At the end of the day we want to know what this movement consists in, and we want it terms as basic as possible. This is where the confusing phrase from Aquinas comes in, for he says that “motion is the act of that which is in potentiality, as such.”[1] In this phrase Aquinas is abbreviating a slightly-less-confusing phrase from Aristotle who says that “change is the actuality of that which exists potentially, in so far as it is potentially this actuality.”[2]

To see what these two are getting at, return to the example of the cup’s resting on the table a meter above the ground. At any given moment, there are two senses in which this potential of the cup’s might be being actualised: first, by the cup actually resting on the table a meter above the ground and second, by me currently being in the process of picking the cup off the ground and placing it on the table. We might put it like this, given that I’ve started this process I’m eitherfinished it (the first case) or I’m still doing it (the second case). In both cases the cup’s potential for resting on the table a meter above the ground is being actualised, but only in the second case is this actualisation an instance of movement. In the first case the cup is sitting on the table a meter above the ground; in the second case it’s not there yet, but it’s on it’s way there. Put (rather verbosely) in terms of act and potency, in the first case the cup’s potential for resting on the table is being actualised and the cup is actually resting on the table, whereas in the second case the cup’s potential for resting on the table is being actualised and the cup is merely potentially resting on the table.

More generally (and symbolically), if we’re considering some object X that has some potential for P currently being actualised, then either X is actually P or X is potentially P. In the former case there is no movement toward P, since X is already P. In the latter case there is movement towards P, since the only way X can have this potential currently being actualised and not be there yet is if X is on its way to P. In the above example X is the cup, and P is “resting on the table a meter above the ground”.

This, then, is what Aquinas and Aristotle are getting at: an actualisation of a potential is movement when, and only when, the thing being actualised is still potentially at its end. Or, more succinctly, movement is the actualisation of a potential while it is still potential.

Notes

1. Summa Contra Gentiles Ch 13
2. Physics 3.1 201a10-12