In this paper I discuss Thomas Aquinas' views about infinite regresses, primarily why he allows some and rejects others. I will first discuss two of the Five Ways from ST I, 2, 3 in order to illustrate the way Aquinas uses infinite regress arguments. Then, I will look at an example of a regress argument that Aquinas rejects. In order to get clear about why Aquinas rejects some regresses I will characterize a distinction to separate acceptable from unacceptable regresses as St. Thomas does. Finally, I will suggest some reasons to explain why Aquinas rejects those regresses that he does reject.
In ST I, 2, 3 Aquinas gives his Five Ways to prove that God exists. The first argument begins by acknowledging the empirical certainty that some things are in motion. Whatever is moved is moved by another, Thomas holds. If that by which something is moved ... be itself moved, then this also must needs be moved by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and consequently, no other mover, seeing that subsequent movers move only inasmuch as they are moved by the first mover ... (ST I, 2, 3). 1
We can see this principle in action when we consider a staff that moves only because it is moved by a hand. Aquinas concludes