Medieval Philosophy

by Peter S. Adamson

never begin. Thus, there is a first mover which, Aquinas blithely adds, “everyone understands to be God.” You see what I mean: the argument has more holes than the plot of a movie about the invention of Swiss cheese. How do we know that something can’t move itself, as when I get up from the sofa to fetch a drink from the kitchen? And why isn’t it possible to have a chain of moved causes, each of which is moved by the previous cause, into infinity? Or take the third of the five ways, which is probably the most mystifying. Here Aquinas asks whether it could be the case that all things are merely possible or contingent. No, because a contingent thing can fail to exist. So, if all things are contingent, then at some point everything would have failed to exist. But that is absurd, because if everything had been non-existent in the past, then nothing would exist now. It follows that not everything is contingent. Instead, there must be a necessary being, and, again, this is what everyone understands to be God.8 The argument looks almost painfully bad, especially the bit where he infers that if each thing could fail to exist, then at some point all things must fail to exist. Aquinas may be assuming as an unstated premise something called the “principle of plenitude”: that every genuine possibility is realized at some point.9 If it is genuinely possible for all things to be non-existent, as surely it is if each thing individually can be non-existent, then at some point this must ha...