1 For a rejection of the Third Way coming from someone almost entirely critical of Aquinas on the topic of arguments for God's existence, see Kenny, Anthony, The Five Ways (London, 1969)Google Scholar, Ch. IV. According to Kenny, Aquinas's Third Way is vitiated by an elementary logical fallacy, by some incredible views about perishing, and by a failure to prove the existence of God as opposed to something else. For a rejection of the Third Way coming from a well known admirer of Aquinas, see Wippel, John F., The Metaphysical Thought of Thomas Aquinas (Washington, D.C., 2000)Google Scholar, Ch. XII. According to Wippel, Aquinas's argument has virtues. Like Kenny, however, Wippel concludes that it is logically flawed.

2 This is the translation of the Third Way offered by Timothy McDermott in Aquinas: Selected Philosophical Writings (Oxford and New York, 1993), p.201.I see no reason to quarrel with it, though I prefer ‘efficient causes’ to McDermott's ‘agent causes’ (the Latin is causis efficientibus), as I indicate in the parenthesis to be found in my quotation from McDermott's translation. In the sentence beginning ‘But just as we proved …’ Aquinas is referring back to the second of his Five Ways.

3 For some discussion of the Latin text of the Third Way, see Steenberghen, Fernand Van, Le Problème de I 'Existence de Dieu duns les Écrits de S. Thomas D'Aquin (Louvain La Neuve, 1980), pp. 188 fGoogle Scholar.

4 Leibniz, Gottfried Wilhelm, Die philosophischen Schriften, ed. Gerhardt, C.I., 7 vols, (Berlin, 1857–90), V01.111, p.400Google Scholar.

5 For an account of Leibniz on God's existence, see Craig, William Lane, The Cosmological Argument from Pluto to Leibniz (London, 1980). Chapter 8CrossRefGoogle Scholar. Those interested in detailed discussion of Leibniz on necessary and contingent being might consult Mates, Benson, The Philosophy of Leibniz (Oxford and New York, 1986).Google Scholar

6 Hepburn, R.W, Christianity and Paradox (London, 1958), p.171Google Scholar. Other writers interpreting the Third Way along Hepburn's lines are quoted in Patterson Brown, ‘St Thomas' Doctrine of Necessary Being’, reprinted in Kenny, Anthony (ed.), Aquinas: A Collection of Critical Essays (London and Melbourne, 1969)CrossRefGoogle Scholar. Brown's paper clearly indicates why they are wrong in their interpretation.

7 Cf. Summa Theologiae Ia,2,1 and Summa Contra Gentiles 1,10. Aquinas holds that there is an inherent contradiction in ‘God does not exist’ since he holds that God's essence is to be (that God is ipsum esse subsistens) and since he takes this to mean that what we succeed in alluding to when we speak of God cannot be something the nature of which is potentially non‐existent. But he does not think that ‘God does not exist’ can be proved to be contradictory apart from the supposition that God, in fact, exists

8 Things can come to be in the world which would not be ‘possible’ beings as Aquinas is thinking of ‘possible beings’ in the Third Way. I and my friends might make a car, but a car, for Aquinas, is not a natural entity. Strictly speaking, it is a collection of things brought together by art. When Aquinas speaks of possible things in the Third Way he is thinking of what is generable in nature — babies, plants, etc. He is thinking of what he would have called entia per se as opposed to entia per accidens.

9 Cf. Summa Contra Gentiles I,10. For a clear explanation of why readings of the Third Way such as that provided by Hepburn are wrong, see Brown, Patterson, ‘St Thomas' Doctrine of Necessary Being’, The Philosophical Review LXXIII (1964)Google Scholar.

10 Cf. Summa Contra Gentiles II,30: ‘Although all things depend on the will of God as first cause, who is subject to no necessity in His operation except on the supposition of His intention, nevertheless absolute necessity is not on this account excluded from things, so as to compel us to say that all things are contingent… Some things are so created by God that there is in their nature a potentiality to non‐being; and this results from the fact that the matter present in them is in potentiality with respect to another form. On the other hand, neither immaterial things, nor things whose matter is not receptive of another form, have potentiality to non‐being, so that their being is absolutely and simply necessary’. (I quote from James F. Anderson's translation of Summa Contra Gentiles, Book 11, Notre Dame and London, 1975Google Scholar).

11 If this is his argument, then he might be read as presenting an argument which can be formally stated so as to include what logicians refer to as a ‘quantifier shift fallacy’. He would be arguing that, if there is a time at which everything perishable perishes, then there is a time when everything perishable has perished. Formally speaking, this argument is invalid and can be compared to ‘All roads lead somewhere, so there is some (one) place (e.g. Rome) to which all roads lead’. My argument in this article, however, is that Aquinas's argument is not to be read along these lines. Be that as it may, though, note that being able to present an argument by means of an invalid form is no proof of its invalidity. Any two premise argument of the form ‘P, Q, therefore R’ is formally invalid. But it is surely valid to argue: ‘If all persons are mortal, and if Mary is a person, then Mary is mortal’, which can be represented as ‘P, Q, therefore R’. We can even make the ‘All roads lead somewhere’ argument come out as valid on a suitable interpretation. As my colleague Gyula Klima has pointed out to me, ‘Every road leads to a place, therefore there is a place to which every road leads’ is formally invalid; but if we know that, on the intended interpretation of the terms ‘road’ and ‘place’ and ‘leads to’ in a particular context, the phrase ‘a place’ has to refer to Rome, then we can conclude that under this specific interpretation the inference is materially valid and can be turned into a formally valid inference by explicating the intended interpretation and by adding ‘and that place is Rome’ to the premise.

12 Cf. his text De Aeternitate Mundi.

13 The Guide of the Perplexed (Translated with an Introduction and Notes by Shlomo Pines, Chicago, 1963), 11. 1.

14 In De Caelo, I, lect.29, n.283.

15 Cf. Wippel, John F., Metaphysical Themes in Thomas Aquinas (Washington, D.C., 1984)Google Scholar, Chapter VIII.

16 Summa Contra Gentiles I.15. I quote from Anton Pegis's translation of Summa Contra Gentiles I (Notre Dame, 1975)Google Scholar.

17 In generous correspondence with me, John Wippel observes that, unlike its parallel in the Summa Contra Gentiles, Aquinas's Third Way does not explicitly claim that a thing that is able to be or not to be depends in some way for its being there on something else (i.e. that the Third Way is not, in its first part, invoking the notion of efficient causation). Yet, it is clear from what Aquinas writes generally that he regards things able to be or not to be as causally dependent for their existence, which inclines me to suggest that we are well within our rights in taking him to be reasoning causally in the first part of the Third Way. Professor Wippel thinks it best to read the Third Way with no reference to texts such as Summa Contra Gentiles 1.15. I think it perfectly proper to allow such texts to interpret the Third Way for us. Professor Wippel thinks that there are temporal referents in the Third Way (because of Aquinas's use of the words quandoque and aliquando) which suggest that Aquinas is offering an essentially different argument from the one in the Contra Gentiles. But I see no reason for taking the Third Way's words quandoque and aliquando in a seriously temporal sense so as to call into question my reading of the Way. Aquinas says that something able to be or not to be quandoque non est (at some time is not). All this need be taken to mean is that something able to be or not to be is something which comes to be. A question to ask then is ‘How does it come to be?’. And I take Aquinas to be assuming in the Third Way that it comes to be by virtue of something else. In the Third Way Aquinas says that, if everything is able to be or not to be, aliquando fuit in rebus (‘once upon a time there was nothing’). But he cannot mean by this that there might have been a time when there was nothing, since he takes time to be a measure of change, since he takes all created things to be changeable, and since he cannot, therefore, have believed in a time without anything. The meaning of aliquando fuit in rebus in the Third Way is surely along the lines: ‘There would not be anything’.

18 I quote from the translation of the Compendium Theologiae translated by Cyril Vollert SJ (St. Louis, Mo, and London, 1949).

19 For a concise account of Aquinas on perishing (written with an eye on his teaching that the human soul cannot perish), see Herbert McCabe, ‘The Immortality of the Soul’, in Anthony Kenny (ed.), Aquinas: A Collection of Critical Essays.

20 Those who favour this reading frequently find Aquinas to be guilty of the ‘quantifier shift fallacy’ to which I refer in Note 11. For they take him to be arguing: ‘If there is a time before which everything able to be or not to be is (was) not, then there is (was) a time before which everything able to be or not to be is (was) not’. But I do not find reason to suppose that Aquinas's Third Way is arguing along these lines, for reasons which I give below. At this point I would also ask the reader to bear in mind what I say about validity in Note 11.

21 Summa Theologiae Ia,46,2 ad 7.

22 This position of Aquinas is much in evidence in what he teaches about God's simplicity. Cf. especially Summa Theologiae, Ia,3,4.

23 Aquinas holds that there can (theoretically) be an infinite series of causes in that, for example, there can (theoretically) be an infinite number of ancestors for any given person. But there cannot, he thinks, be an infinite series of causes where the effect in question is the sheer existence of something. He holds that a world consisting only of contingent particular causes needs the concurrent activity of a universal cause for their existence and activity at all times, for there would otherwise be nothing to account for them being in the first place. That seems a cogent position to me, but one's response to it will depend on the extent to which one is struck by the question ‘How come something rather than nothing?’. This is not the place to address that question. I have attempted to do so in, for example, ‘The Mystery of God: Aquinas and McCabe’(New Blackfriars (July‐August 1996) and ‘Aquinas, God and Being’(The Monist, 1998).

24 John Duns Scotus, Reportatio I A. I quote from Frank, William A. and Wolter, Allan B., Duns Scotus, Metaphysician (West Lafayette, Ind, 1995), p. 43Google Scholar.

25 For comments on earlier versions of this paper I am very grateful to my colleagues Gyula Klima and Brian Leftow. As I have noted, I am also grateful for comments from John Wippel.