Aquinas’s Parasitic Cosmological Argument

Aquinas’s Parasitic Cosmological Argument

MacDonald, Scott

Medieval Philosophy and Theology, vol. 1 (1991)


In Summa theobgiae (ST) 1.2.3 Aquinas says that his first way of proving the existence of God is clearer (manifestior), seemingly meaning that it is the clearest of the five ways he will offer. Most philosophers who have considered the matter, however, have disagreed. The proof from motion has been almost entirely abandoned, and philosophers from Clarke and Leibniz to Rowe and Swinburne have preferred versions of the cosmological argument nearer to Aquinas’s second or third ways. One reason for the neglect of the first way is that it has been generally supposed to be subject to several obvious, devastating criticisms, among them that it crucially depends on archaic physical theory, ancient astrology, and one or more elementary fallacies.

In this paper I argue that the proof from motion can be freed from the trappings of ancient science and astrology and defended against the most common of the strictly philosophical criticisms of it. Having defended the argument against some well-known criticisms, I argue that it nevertheless fails as an independent proof for God’s existence because it depends for its validity on another of Aquinas’s proofs for God’s existence. Commentators have not adequately appreciated the significance of the parasitical nature of the proof from motion, though Aquinas himself did, as I argue in the final section of this essay.

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