‘Sometime a paradox’, now proof: Yablo is not first order

Interesting as they are by themselves in philosophy and mathematics, paradoxes can be made even more fascinating when turned into proofs and theorems. For example, Russell’s paradox, which overthrew Frege’s logical edifice, is now a classical theorem in set theory, to the effect that no set contains all sets. Paradoxes can be used in proofs of some other theorems—thus Liar’s paradox has been used in the classical proof of Tarski’s theorem on the undefinability of truth in sufficiently rich languages. This paradox (as well as Richard’s paradox) appears implicitly in Gödel’s proof of his celebrated first incompleteness theorem. In this paper, we study Yablo’s paradox from the viewpoint of first- and second-order logics. We prove that a formalization of Yablo’s paradox (which is second order in nature) is non-first-orderizable in the sense of George Boolos (1984). This was sometime a paradox, but now the time gives it proof. —William Shakespeare (Hamlet, Act 3, Scene 1).

Logic, Language, and Mathematics

This Festschrift volume contains a series of specially commissioned papers by leading philosophers on themes from the philosophy of Crispin Wright and a previously unpublished paper by George Boolos, together with a substantial set of replies by Wright. Section I consists of five essays on Wright’s Neo-Fregean approach in the philosophy of mathematics, Section II consists of two essays on Wright’s work on vagueness, intuitionism and the Sorites Paradox, Section III contains two essays on logical revisionism, and Section IV consists of a single essay on the epistemology of metaphysical possibility. The volume also contains a full bibliography of Wright’s philosophical publications.

GENERALIZING BOOLOS’ THEOREM

It’s well known that it’s possible to extract, from Frege’s Grudgesetze, an interpretation of second-order Peano Arithmetic in the theory $H{P^2}$, whose sole axiom is Hume’s principle. What’s less well known is that, in Die Grundlagen Der Arithmetic §82–83 Boolos (2011), George Boolos provided a converse interpretation of $H{P^2}$ in $P{A^2}$. Boolos’ interpretation can be used to show that the Frege’s construction allows for any model of $P{A^2}$ to be recovered from some model of $H{P^2}$. So the space of possible arithmetical universes is precisely characterized by Hume’s principle.In this paper, I show that a large class of second-order theories admit characterization by an abstraction principle in this sense. The proof makes use of structural abstraction principles, a class of abstraction principles of considerable intrinsic interest, and categories of interpretations in the sense of Visser (2003).