Ex Possibili et Necessario: A Re-Examination of Aquinas’s Third Way

EX POSSIBILI ET NECESSARIO: A RE-EXAMINATION OF AQUINAS'S THIRD WAY THOMAS A. F. KELLY St. Patrick's College Maynooth, Ireland !SHALL NOT ATTEMPT in this essay any systematic exegesis of Thomistic texts, still less an interpretation of Aquinas on arguments for the existence of God. What I shall try to do, rather, is to suggest that a valid argument for the existence of God, resting upon premises known to be true-which is what I would call a proof 1-results from a restructuring of the Third Way. This may seem a large, not to say ill-advised, claim, in a time when even proponents of theism tend not to stress the argumentatious underpinning of their position, and when there seems to be a kind of floating opinion, shared by many, that such argument is useless or even harmful. However, my answer to such misgivings is the only possible one, namely, that if the argument works, that is sufficient reason for considering it. The restructured argument, though clearly not identical to the Third Way, nevertheless makes use of the metaphysical principles stated in it. Since in pursuing this investigation I am following what I take to be the spirit of Aquinas, namely, the desire to know not merely what philosophers-himself in particular -have said, but, more profoundly, where the truth of the matter under discussion lies, I shall make use of the logical and linguistic developments that have taken place since his time. 1 Proof, in the present writer's view, has little or nothing to do with conviction on the part of those who read or hear it; if we are both honest and rational, conviction will undoubtedly follow our understanding of the logical status of the argument, but this is not a contributory factor to that status. For a contrasting view, see A Companion to Epistemology, ed. Jonathan Dancy and Ernest Sosa (Oxford: Blackwell, 1993), under "proof." 63 64 THOMAS A. F. KELLY I. THE QUESTION In making this attempt, it would be well to begin by bringing Aquinas's text before our eyes. The third way is based on what need not be and what must be, and runs as follows. Some of the things we come across can be but need not be, for we find them springing up and dying away, thus sometimes in being and sometimes not. Now everything cannot be like this, for a thing that need not be, once was not; and if everything need not be, once upon a time there was nothing. But if that were true there would be nothing even now, because something that does not exist can only be brought into being by something already existing. So that if nothing was in being nothing could be brought into being, and nothing would be in being now, which contradicts observation. Not everything therefore is the sort of thing that need not be; there has got to be something that must be. Now a thing that must be, may or may not owe this necessity to something else. But just as we must stop somewhere in a series of causes, so also the series of things which must be and owe this to other things. One is forced therefore to suppose something which must be, and owes this to no other being than itself; indeed it itself is the cause that other things must be.' The argument seems to fall naturally into two parts, the first ending with the conclusion that there has to be something that must be, and the second with the conclusion that this something owes its necessity only to itself, and is the cause of other things' having to be. To make this division more obvious, I have split the argument into two paragraphs. Various objections may be raised to each part, the most obvious to the first being that from the premise that everything at some time is not, it is fallacious to conclude that at some time nothing exists, and, to the second, that it is false that one must stop somewhere in a series of causes. It will become clear that, as it turns out, neither of these objections...