3.2 Argument Two
Aquinas’s second SCG 1,13 argument for “God exists” is another argument from motion inspired by what he takes Aristotle to maintain. Here is how it goes:
1. If “Every mover is moved” is a true proposition, then it is true “either by itself or by accident.”
2. If “by accident,” then the proposition is not necessarily true, and it might be true that no mover is moved.23
3. Someone might argue that if a mover is not moved, it does not move, thereby concluding that it is possible that nothing is moved since if nothing moves, nothing is moved.
4. But this implies what Aristotle takes to be impossible: that there is no motion at any time.
5. Therefore, if “Every mover is moved” is true, it is not so “by accident” since a proposition that is false and impossible does not follow from a proposition that is possibly true but actually false.24
I should note that Aquinas immediately defends the same conclusion as the above argument arrives at in a different way. Here is his alternative argument:
1. Something can have two accidental features without having them of necessity.25
2. If something happens to be accidentally (per accidens) both a mover and a thing moved, there could probably be an unmoved mover (just as there might probably be something lacking one of any two accidental features).26
3. So, if being a mover and being moved is what something is only accidentally, and if something else can be moved without it being a mover, it is probable that there is a mover that is not moved.
This argument is offered by Aquinas not as a demonstration but only as providing some support for the conclusion that “Every mover is accidentally moved by another” is false. The argument is suggesting that if “being a mover” and “being moved” do not both have to be predicated of (ascribed to) something of necessity, then there could be a mover that is unmoved.
Yet Aquinas also thinks that it is not necessarily true (or in his language, true per se) that every mover is moved by another, and here is his argument for this conclusion.
1. If “Every mover is moved by another” is necessarily true, the mover must be moved either by “the same kind of motion” as that by which it moves, or “by another.”27
2. If every mover is moved by the same kind of motion, then, for example, a “cause of alteration must itself be altered” or “a healing cause must itself be healed” or “a teacher must be taught” with respect to the same knowledge.
3. But this is impossible. If it were true, “the same thing would be possessed and not possessed by the same being.”
4. If every mover is moved by a different kind of motion, since the kinds of motion are finite in number “it will follow that we cannot proceed to infinity” and there will have to be a first mover that is not moved by another.28
But need a first mover that is not moved by something else be absolutely unmoved? Aquinas raises this question and says that the answer to it is no. For something might be moved by itself (a point he has already made in SCG 1,13). If it is, though, Aquinas adds, the thing in question cannot be moved entirely by itself as a whole; it would have to be moved by a part or parts of itself, meaning that a first mover must ultimately be moved by what is absolutely unmoved if there cannot be an infinite regress in things moving and being moved. In this sense, there has to be an unmoved source of movement even when it comes to things, such as people, that move themselves. Indeed, there has to be such a source whether or not the things in question come to be and pass away over time considered as lacking beginning or end. It is “necessary either to arrive immediately at an unmoved separate first mover, or to arrive at a self-moved mover from whom, in turn, an unmoved separate first mover is reached.”29