Logic Without Syntax

Abstract

This paper presents an abstract, mathematical formulation of classical propositional logic. It proceeds layer by layer: (1) abstract, syntax-free propositions; (2) abstract, syntax-free contraction-weakening proofs; (3) distribution; (4) axioms (p OR NOT p). Abstract propositions correspond to objects of the category G(Rel^L) where G is the Hyland-Tan double glueing construction, Rel is the standard category of sets and relations, and L is a set of literals. Abstract proofs are morphisms of a tight orthogonality subcategory of Gl(Rel^L), where we define Gl as a lax variant of G. We prove that the free binary product-sum category (contraction-weakening logic) over L is a full subcategory of Gl(Rel^L), and the free distributive lattice category (contraction-weakening-distribution logic) is a full subcategory of Gl(Rel^L). We explore general constructions for adding axioms, which are not Rel-specific or (p OR NOT p)-specific.