Which ‘Intensional Paradoxes’ are Paradoxes?

Abstract

AbstractWe begin with a brief explanation of our proof-theoretic criterion of paradoxicality—its motivation, its methods, and its results so far. It is a proof-theoretic account of paradoxicality that can be given in addition to, or alongside, the more familiar semantic account of Kripke. It is a question for further research whether the two accounts agree in general on what is to count as a paradox. It is also a question for further research whether and, if so, how the so-called Ekman problem bears on the investigations here of the intensional paradoxes. Possible exceptions to the proof-theoretic criterion are Prior’s Theorem and Russell’s Paradox of Propositions—the two best-known ‘intensional’ paradoxes. We have not yet addressed them. We do so here. The results are encouraging. §1 studies Prior’s Theorem. In the literature on the paradoxes of intensionality, it does not enjoy rigorous formal proof of a Gentzenian kind—the kind that lends itself to proof-theoretic analysis of recondite features that might escape the attention of logicians using non-Gentzenian systems of logic. We make good that lack, both to render the criterion applicable to the formal proof, and to see whether the criterion gets it right. Prior’s Theorem is a theorem in an unfree, classical, quantified propositional logic. But if one were to insist that the logic employed be free, then Prior’s Theorem would not be a theorem at all. Its proof would have an undischarged assumption—the ‘existential presupposition’ that the proposition $$\forall p(Qp\!\rightarrow \!\lnot p)$$ ∀ p ( Q p → ¬ p ) exists. Call this proposition $$\vartheta $$ ϑ . §2 focuses on $$\vartheta $$ ϑ . We analyse a Priorean reductio of $$\vartheta $$ ϑ along with the possibilitate $$\Diamond \forall q(Qq\!\leftrightarrow \!(\vartheta \!\leftrightarrow \! q))$$ ◊ ∀ q ( Q q ↔ ( ϑ ↔ q ) ) . The attempted reductio of this premise-pair, which is constructive, cannot be brought into normal form. The criterion says we have not straightforward inconsistency, but rather genuine paradoxicality. §3 turns to problems engendered by the proposition $$\exists p(Qp\wedge \lnot p)$$ ∃ p ( Q p ∧ ¬ p ) (call it $$\eta $$ η ) for the similar possibilitate $$\Diamond \forall q(Qq\!\leftrightarrow \!(\eta \!\leftrightarrow \! q))$$ ◊ ∀ q ( Q q ↔ ( η ↔ q ) ) . The attempted disproof of this premise-pair—again, a constructive one—cannot succeed. It cannot be brought into normal form. The criterion says the premise-pair is a genuine paradox. In §4 we show how Russell’s Paradox of Propositions, like the Priorean intensional paradoxes, is to be classified as a genuine paradox by the proof-theoretic criterion of paradoxicality.