Edward Feser, Aquinas: A Beginner’s Guide (Oxford: Oneworld, 2009), 95-98.

At this point the critic of the Third Way might think to challenge the premise that “that which does not exist only begins to exist by something already existing,” so as to undermine the claim that if there was ever a time when nothing existed, then nothing would exist now. But this premise is just a variation on the principle of causality, and we have already seen how that principle might be defended. A more promising strategy for the critic might seem to be to suggest (as J. L. Mackie does) that even if individual contingent things all go out of existence, there might still be some underlying stuff out of which they are made (a “permanent stock of matter,” in Mackie’s words) which persists throughout every generation and corruption. Now if this were so, then what would follow, given the Aristotelian conception of necessity we’ve been describing, is that this stock of material stuff would itself count as a necessary being. But (so the suggestion continues) the critic could happily accept this (as Mackie does) given that such a “necessary being” would, in view of its material nature, clearly not be divine.

The trouble with this reply, though, is that it falsely purports to be asserting something that Aquinas would deny. In fact, surprising as it might seem, Aquinas would be quite happy, at least for the sake of argument, to concede that the material world as a whole might be a kind of necessary being, in the relevant sense of being everlasting or non-transitory. After all, as we have repeated many times, Aquinas does not think that proving the existence of God requires showing that the material world had a beginning. Moreover . . . Aquinas himself insists that while individual material things are generated and corrupted, matter and form themselves are (apart from special divine creation, to which he would not appeal for the purposes of the argument at hand lest he argue in a circle) not susceptible of generation and corruption. Far from regarding the notion of the material world as necessary as a blow to the project of the Third Way, Aquinas would in fact regard it as a vindication of his claim that there must be a necessary being. Indeed, he recognizes the existence of other non-divine necessary beings as well, such as angels and even heavenly bodies (which, given the astronomical knowledge then available, the medievals mistakenly regarded as not undergoing corruption).

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 immediately 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 involves a 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 whether 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 it is the sort of thing that could possibly have its necessity in itself, or whether instead it must derive its necessity from something else, from something which keeps it in existence everlastingly.

It is immediately 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 constitute a necessary being having of itself its own necessity. For one thing, and as we have already noted, 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 be said to be everlasting (and prime matter, for the reasons just given, cannot have its everlastingness of itself). Second, even if there could 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 . . . 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 from 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.

The essence/existence distinction also implies that other sorts of non-divine necessary beings, such as angels (which on Aquinas’s view are composites of a pure form together with an act of existing), would have to derive their necessity from something else. The only thing that could stop an explanatory regress of necessary beings would therefore be something whose essence and existence are identical, and who is a necessarily existing being precisely because it just is subsistent being or existence. Here we need only refer back to the “existential proof” considered when discussing the Second Way in order to fill in the details; and the upshot is that the Second Way and Third Way appear to converge on exactly the same being, albeit they do so from very different starting points (and thus remain distinct arguments).

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Return to Lesson 8: The Difference Between Necessary and Possible Beings