Implementing cantor’s paradise

Set-theoretic paradoxes have made all-inclusive self-referential Foundational Theories almost a taboo. The few daring attempts in the literature to break this taboo avoid paradoxes by restricting the class of formula allowed in Cantor’s naïve Comprehension Principle. A different, more intensional approach was taken by Fitch, reformulated by Prawitz, by restricting, instead, the shape of deductions, namely allowing only normal(izable) deductions. The resulting theory is quite powerful, and consistent by design. However, modus ponens and Scotus ex contradictione quodlibet principles fail. We discuss Fitch-Prawitz Set Theory (FP) and implement it in a Logical Framework with so-called locked types, thereby providing a “Computer-assisted Cantor’s Paradise”: an interactive framework that, unlike the familiar Coq and Agda, is closer to the familiar informal way of doing mathematics by delaying and consolidating the required normality tests. We prove a Fixed Point Theorem, whereby all partial recursive functions are definable in FP. We establish an intriguing connection between an extension of FP and the Theory of Hyperuniverses: the bisimilarity quotient of the coalgebra of closed terms of FP satisfies the Comprehension Principle for Hyperuniverses.