Nominalism, Trivialism, Logicism

Abstract

This paper is an effort to extract some of the main theses in the philosophy of mathematics from my book, The Construction of Logical Space. I show that there are important limits to the availability of nominalistic paraphrase-functions for mathematical languages, and suggest a way around the problem by developing a method for specifying nominalistic contents without corresponding nominalistic paraphrases.Although much of the material in this paper is drawn from the book-and from an earlier paper (Rayo 2008)-I hope the present discussion will earn its keep by motivating the ideas in a new way, and by suggesting further applications. NominalismMathematical Nominalism is the view that there are no mathematical objets. A standard problem for nominalists is that it is not obvious that they can explain what the point of a mathematical assertion would be. For it is natural to think that mathematical sentences like 'the number of the dinosaurs is zero' or '1 + 1 = 2' can only be true if mathematical objects exist. But if this is right, the nominalist is committed to the view that such sentences are untrue. And if the sentences are untrue, it not immediately obvious why they would be worth asserting. * For their many helpful comments, I am indebted to Vann McGee, Kevin Richardson, Bernhard Salow and two anonymous referees for Philosophia Mathematica. I would also like to thank audiences at Smith College, the Università Vita-Salute San Raffaele, and MIT's Logic, Langauge, Metaphysics and Mind Reading Group. Most of all, I would like to thank Steve Yablo. 1A nominalist could try to address the problem by suggesting nominalistic paraphrases for mathematical sentences. She might claim, for example, that when one asserts 'the number of the dinosaurs is zero' one is best understood as making the (nominalistically kosher) claim that there are no dinosaurs, and that when one asserts '1 + 1 = 2' one is best understood as making the (nominalistically kosher) claim that any individual and any other individual will, taken together, make two individuals.1 Such a strategy faces two main challenges. The first is to explain why mathematical assertions are to be understood non-standardly. One way for our nominalist to address this challenge is by claiming that mathematical assertions are set forth 'in a spirit of makebelieve' (Yablo 2001). She might argue, in particular, that when one makes a mathematical assertion one is, in effect, claiming that the asserted sentence is true in a fiction, and more specifically a fiction according to which: (a) all non-mathematical matters are as in reality, but (b) mathematical objects exist with their standard properties. This proposal leads to the welcome result that fictionalist assertions of mathematical sentences can convey information about the real world. For instance, one can use a fictionalist assertion of 'the number of the dinosaurs is zero' to convey the information that there are no dinosaurs, since the only way for 'the number of the dinosaurs is zero' to be true in a fiction whereby mat...