¹ In a book on the Five Ways under contract with Cambridge University Press, Reading Aquinas’s Five Ways, I will offer a longer treatment of the Fifth Way. My thoughts are abridged here to fit this article.

² Quinta via sumitur ex gubernatione rerum. Videmus enim quod aliqua quae cognitione carent, scilicet corpora naturalia, operantur propter finem, quod apparet ex hoc quod semper aut frequentius eodem modo operantur, ut consequantur id quod est optimum; unde patet quod non a casu, sed ex intentione perveniunt ad finem. Ea autem quae non habent cognitionem, non tendunt in finem nisi directa ab aliquo cognoscente et intelligente, sicut sagitta a sagittante. Ergo est aliquid intelligens, a quo omnes res naturales ordinantur ad finem, et hoc dicimus Deum.

Summa theologiae Ia q.2 a.3 resp., in Sancti Thomae Aquinatis Opera omnia iussu Leonis XIII, ed. by the Leonine Commission, vols. 4–12 of 50 vols. (in preparation) (Rome: 1882–). Translated from the Latin by The Fathers of the English Dominican Province as Summa Theologica (New York: Benzinger Bros., 1947). I have also consulted Alfred Freddoso’s translation of the Summa in progress on his personal website at https://www3.nd.edu/~alfreddos/. I generally follow a cited English translation of a work by Aquinas but include frequent changes of my own. I mostly use the Latin editions of Aquinas’s works available in the Corpus Thomisticum online at www.corpusthomisticum.org.

³ The roots of the Fifth Way in Physics II c.8 and these points made by Aristotle are noted by Michael Augros in his Aquinas on Theology and God’s Existence: The First Two Questions of the Summa Theologiae Newly Translated and Carefully Explained (Heusenstamm, Germany: Editiones Scholasticae, 2019), p. 411.

⁴ Here I follow Laurence Shapcote (1864–1947) in his translation of the Summa theologiae credited to the English Dominican Fathers, where he uses ‘intelligence’ for the first two instances of cognitio in the Fifth Way and ‘knowledge and intelligence’ (cognoscent et intelligente) near the end. John Owens notes that this underscores that the being in question has something like an intellect with understanding able to grasp essences and ends. Aquinas thinks that nonrational animals, such as a chimpanzee, have a kind of basic awareness. But this obviously does not fit the type of superior intellect needed for the conclusion. John Owens, ‘Aquinas’ Fifth Way’, New Blackfriars, 101 (2020), 726–39, see pp. 727–28. My focus in this article on the teleological scope of the Fifth Way is partly indebted to Owens.

⁵ Aquinas in Summa contra Gentiles III c.2 wants to establish the thesis that ‘in acting, every agent intends an end’. There he gives multiple arguments that there cannot be an infinite regress of final causes. He offers four such arguments in his Commentary on Aristotle’s Metaphysics II l.4 n.1–3.

⁶ See also Summa theologiae I-IIae q.1 a.2 resp., I-IIae q.6 a.1 resp.

⁷ See for instance, John Wippel, The Metaphysical Thought of Thomas Aquinas (Washington, DC: Catholic University of America Press, 2000), p. 480. Thomas Gilby sees the teleological observation opening the Fifth Way extending ‘to all creatures … all things that do not of themselves possess their ends’. In ‘The Fifth Way’, p. 207, which forms Appendix 5 to volume 2 (pp. 206–08) of the Blackfriars edition of the Summa Theologiae, ed. by Thomas Gilby et al., 61 vols. (London: Eyre and Spottiswoode; New York: McGraw-Hill Book Company, 1964–1981).

⁸ Contra Gentiles III c.2, in Sancti Thomae Aquinatis Opera omnia iussu Leonis XIII, ed. by the Leonine Commission, vols. 13–15 of 50 vols. (in preparation) (Rome: 1882–). Trans. by Anton Pegis, James Anderson, Vernon Bourke, and Charles O’Neil as On the Truth of the Catholic Faith, 5 vols (New York: Doubleday, 1955–57), reprinted as Summa Contra Gentiles (Notre Dame: University of Notre Dame Press, 1975). I have made changes to the translation where it is used.

Relevant discussions of this principle, often phrased as ‘every agent acts for an end’, occur in Contra Gentiles III c.1–3, 16–20 and in Summa theologiae Ia-IIae q.1 a.2. A classic treatment is George Klubertanz’s, ‘St. Thomas’ Treatment of the Axiom, Omne Agens Propter Finem’, in An Etienne Gilson Tribute, ed. by C.J. O’Neil (Milwaukee: Marquette University Press, 1959), pp. 101–17.

⁹ A point developed by Michael Augros, Aquinas on Theology, p. 423.

¹⁰ Quaestiones disputatae De veritate q.1 a.1 resp, in Sancti Thomae de Aquino Opera omnia iussu Leonis XIII, ed. by Leonine Commission, Vols. 22(1/1)–22(3/2) of 50 vols, (Rome: 1882–). Trans. by Robert Mulligan, James McGlynn, and R. Schmidt as On Truth, 3 vols (Chicago: Henry Regnery, 1952–1954). I have made changes to the translation where it is used.

¹¹ The larger context is worth quoting: ‘That which most strongly demonstrates that nature acts for the sake of something is the fact that in the operation of nature a thing is always found to be as better and fitting as it can be. Thus, [for example] the foot is made in a certain way to be suitable for walking’. In octo libros Physicorum Aristotelis expositio II l.12 n.3, ed. by P. Maggiolo (Turin: Marietti, 1954). Translated as Commentary on Aristotle’s Physics, trans. by Richard J. Blackwell, Richard J. Spath, W. Edmund Thirlkel (New Haven: Yale University, 1963). I have made changes to this translation. Michael Augros, Aquinas on Theology, pp. 440–44, explains Aquinas on the optimal good in a way helpful to the present discussion.

¹² Aquinas is arguing in Contra Gentiles II c.39 n.7 that the fact the world is made up of individual substances, distinct from each other, cannot be by chance. ‘But the good and the best in the universe consists in the mutual order of its parts, which cannot be without their distinction from one another; for by this order the universe is constituted in its wholeness, and in this is its optimal good. Therefore, it is this very order of the parts of the universe and of their distinction which is the end of the production of the universe. It remains that the distinction among things is not [just] by chance’.

¹³ Summa theologiae Ia q.19 a.9 resp., q.49 a.2. In Ia q.22 a.2 ad 2 Aquinas says: ‘Hence, corruption and defects in natural things are said to be contrary to a particular nature. Yet, they are in keeping with the universal plan [of nature], insofar the defect in one thing yields to the good of another, or even that of the whole universe. The reason is the corruption of one is the generation of another, through which a species is conserved in existence. Therefore, since God is the universal provider of all being, it belongs to His providence to permit certain defects in particular things, so that the perfect good of the universe may not be impeded. For if all evil were prevented, much good would be absent from the universe. A lion would not live if there were no slaying of animals; nor would the patience of martyrs exist if there were no tyrannical persecution’.

¹⁴ William Ockham, Quaestiones quodlibetales IV q.1–2, in what is thought to be his latest treatment of final causality. Translated as Quodlibetal Questions: Quodlibets 1–7, by Alfred Freddoso and Francis Kelly (New Haven: Yale University Press, 1998). In Quod IV q.1 (p.259) Ockham finds that final causation ‘is inappropriate in the case of natural actions’ and is so ‘only in the case of voluntary ones’.

¹⁵ Michael Augros discusses the the notion of “blind necessity” in Aquinas on Theology, p. 428. See Benignus Gerrity, Nature, Knowledge, and God (Milwaukee: Bruce Publishing, 1947), pp. 87–88. See also Edward Feser, Scholastic Metaphysics: A Contemporary Introduction (Heusenstamm, Germany: Editiones Scholasticae, 2014), pp. 96–97.

¹⁶ Commentary on Aristotle’s Physics II l.14 n.8.

¹⁷ Ia-IIae q.1 a.2 resp. See also Aquinas’s Commentary on the Sentences of Peter Lombard II d.25 q.1 a.1.

¹⁸ The examples are adapted from a source or blend of sources I cannot readily retrace. The second is possibly from Christopher Martin’s God and Explanations, p. 182., and also Edward Feser, Scholastic Metaphysics, p. 88.

¹⁹ In the Commentary on the Sentences, Aquinas says that the actions of natural bodies are ordered to their actions in virtue of being ‘constituted’ to have a nature which acts for an end, in Sentences II d.25 q.1 a.1 solutio. Aquinas goes on to argue that the whole of nature shows itself to be the work of something with intelligence.

²⁰ Sentences II d.25 q.1 a.1 solutio: ‘For the determination of an agent to some particular action, it has to be through some act of knowing presented as an end of that action’. In Scriptum super libros Sententiarum Magistri Petri Lombardi II d.34 q.1 a.1, vol.2 of 4 vols, ed. by R. Mandonnet (vols.1–2) and M. Moos (vols.3–4) (Paris: Lethielleux, 1929–1947). Currently, there is no published English translation of the whole work.

²¹ De veritate q.5 a.2 resp., in an article on ‘Whether the world is ruled by providence’ (utrum mundus providentia regatur). Michael Augros summarizes this type of argument from the good outcomes of natural agents, ‘Many natural actions, such as those of living things, regularly produce many good and functional outcomes (such as useful organs), and any one functional outcome is just a single possibility in an infinite ocean of dysfunctional alternatives. It is therefore impossible that nature not be oriented toward the good’. Aquinas on Theology, p. 438.

²² Examples are borrowed from Christopher Martin, Thomas Aquinas: God and Explanations (Edinburgh: Edinburgh University Press, 1997), pp.184–185. Also helpful is Benignus Gerrity, Nature, Knowledge, and God, pp. 87–99.

²³ Michael Augros, Aquinas on Theology, p. 423.

²⁴ Christopher Martin, God and Explanations, pp.182–183, finds the reasoning more ‘strong’ than ‘conclusive’ in the final steps, after initially entertaining doubts about the effectiveness of the Fifth Way. John Wippel is brief and noncommittal on whether the Fifth Way succeeds, Metaphysical Thought, p.485. Dennis Bonnette, who has written extensively on the Five Ways, endorses the reasoning of the Fifth Way in a recent, brief piece posted on his website, ‘Understanding the Mysterious Fifth Way to God’s Existence’ at www.drbonnette.com. Lubor Velecky finds this last part of the argument makes a plausible case for God, Aquinas’ Five Arguments in the Summa Theologiae 1a 2, 3 (The Netherlands: Pharos, 1994), p. 97.

²⁵ Lubor Velecky, Aquinas’ Five Arguments, p. 94.

²⁶ Summa theologiae I-IIae q.1 a.2 ad 3. In Contra Gentiles III c.64 n.6 there is a succinct argument for a single orderer of nature: ‘Furthermore, things that are different in their natures do not come together into one order unless they are gathered by a single orderer into one unit. But in the whole of what there is, things are distinct and possessed of contrary natures; yet, all come together in one order. Moreover, while some things make use of the actions of others, some are also helped or commanded by others. Therefore, there must be one orderer and governor of the whole of things’.

²⁷ ‘For Aquinas, any name that exclusively designates the divine nature from effects will satisfactorily serve in answering the question whether God exists. All other properties beyond that name or names belong equally to the question what God is’. David Twetten, ‘Clearing a “Way” for Aquinas: How the Proof from Motion Concludes to God’, Proceedings of the American Catholic Philosophical Association, 70 (1996), pp. 259–78, p. 271.

Summa theologiae Ia q.2 a.2 on ‘Whether it can be demonstrated that God exists’ explains this further: ‘When the existence of a cause is demonstrated from an effect, it is necessary that this effect takes the place of the definition of the cause, in proving the cause’s existence, and this is especially so in regard to God. The reason is that, in order to prove the existence of anything, it is necessary to accept as a middle term that which the word signifies, and not what it is [i.e. its essence] (quid significet nomen non autem quod quid est), for the question of its essence follows on the question of its existence. Now the names given to God are derived from His effects, as previously shown; consequently, in demonstrating the existence of God from his effects, we may take for the middle term that which the word “God” signifies. God can be shown to exist by looking at the significance of the term “God”’.

²⁸ F.C. Copleston, Aquinas (New York: Penguin Books, 1955), p. 130.