Aquinas's Metaphysics of Modality: A Reply to Leftow

Abstract

Brian Leftow sets out to provide us with an account of Aquinas’s metaphysics of modality. Drawing on some important recent work, which is surely close to the spirit (if not quite the letter) of Aquinas’s thought, he frames his discussion in terms of “truthmakers”: what is it that makes true claims about possibility and necessity—that is to say, what serves as their ontological ground or ultimate metaphysical explanation? Leftow’s main thesis is that, for Aquinas, all true modal claims are made true by one and the same thing—namely, God. Along the way, however, he also attempts to show that Aquinas’s endorsement of this thesis commits him to an objectionable form of Platonism about possible worlds. Leftow’s discussion is wide-ranging and provocative, and there is much to be learned from it both about metaphysics in general and about Aquinas’s metaphysics in particular. Moreover, I myself am fully prepared to accept his main thesis: it seems to me that Aquinas does think God is the truthmaker for all modal truths. Nonetheless, I have some worries about the argument Leftow uses to establish his thesis, as well as some reservations about his characterization of the view that results from the thesis itself. Aquinas’s account seems to me more plausible and interesting than Leftow’s discussion suggests, and hence capable of being defended against each of the two principal objections he raises. My aim in what follows is to make good on these claims. In order to explain what I think is wrong with Leftow’s interpretation, I begin (in Section I) with a brief summary of the argument for his main thesis, showing precisely how it relates to