That this should not be surprising, and in particular that it should not be regarded as damaging to the aim of proving the existence of God specifically, should be evident when we remember that proving the existence of a necessary being is only one component of the overall argumentative strategy of the Third Way. For recall that at this stage of the argument Aquinas immedately goes on to say that “every necessary thing either has its necessity caused by another, or not” and then argues that a series of necessary beings cannot go on to infinity. This might seem very odd to those contemporary philosophers who think of necessity in terms of possible worlds or who regard all necessity as logical necessity. “How could a necessary being get its necessity from another?” such a philosopher might ask. “It either exists in all possible worlds or it does not, or the assertion of its non-existence either involvesa self-contradiction or it does not. End of story.Certainly there can be no question of anything causing it to exist in all possible worlds or causing it to be logically necessary!” But when we keep in mind that Aquinas does not mean “necessary” in the sense in which such contemporary philosophers understand it, but rather in the sense of “everlasting” or “permanent,” we can see that it makes perfect sense to consider wheather a thing’s necessity is derived or not. In particular, we can see that it is not enough to show that the material universe as a whole (or an angel, a heavenly body, or whatever) is a necessary being in the relevant sense. One also needs to know whether instead it must derive its necessity from something else, from something which keeps it in existence everlastingly.
It is immeditately obvious, however, that matter qua matter cannot possibly have its necessity of itself, at least on an Aristotelian conception. For matter considered apart from anything else, and in particular apart from form, is just “prime matter” or pure potentiality; and pure potentiality, since by definition it has no actuality, has no reality either, necessary or otherwise. Matter exists only insofar as it is combined with substantial form to comprise a substance. Nor would it help the critic of the Third Way to suggest that it is matter and form together that consitute a necessary being having of itself its own necessity. For one thing… … individual material things are constantly going out of existence and thus losing their forms, and it is in their nature to do so. Hence it cannot be any particular material substance, but only prime matter, which can bee said to be everlasting (and prime matter, for the reasons just given, cannot have its everlastingness of itself). Second, even if there coould be some composite of form and matter which exists everlastingly, since in purely material substances form depends on matter just as matter depends on form, we would have (as Martin has pointed out) an explanatory vicious circle unless we appealed to something outside the form/matter composite on which it depends for its existence. Third, since (given Aquinas’s doctrine of essence and existence) the existence of any material thing is distinct form its essence, we would need in any case to appeal to something outside it in order to explain how its essence and existence come together so as to make it real. (Note that this particular point would apply to material things even if, contrary to Aristotle and Aquinas, we did not regard them as composites of form and matter.) There is no way, then, plausibly to hold that matter might have its necessity of itself. Even a “necessarily existing” or everlasting material world would have to depend on something outside it for its existence. And this something could not itself be a composite either of form and matter or essence and existence, on pain of infinite regress."
