A Natural Transformation between the Model Constructions of the Completeness and Compactness Theorems, Enhanced by Rigidity and 2-Categorical Strengthening (2503.16555v1)

Abstract: In this paper we present a mathematically rigorous and constructive framework that unifies two canonical model constructions in classical first order logic. In particular, we define two functors F and G from the category of consistent first order theories to the category of models. The functor F is constructed via the Henkin method, which extends any given theory to a maximal consistent theory by means of a fixed enumeration and the systematic introduction of Henkin constants, and then constructs a term model by taking the quotient of the term algebra with respect to provable equality. The functor G is obtained through a canonical compactness based construction, using either a fixed ultraproduct or a saturation procedure, ensuring that the resulting model is unique up to isomorphism. We prove the existence of a natural transformation eta from F to G such that each component is an isomorphism. Moreover, by leveraging the uniqueness of saturated (or prime) models in countable languages, we show that eta is rigid, meaning any other natural transformation between F and G must equal eta. Furthermore, we establish a strong natural equivalence between F and G in the two categorical sense, with eta and its inverse satisfying the required coherence conditions. This unification not only deepens our understanding of the interplay between proof theory and model theory, but also opens new avenues for applications in automated theorem proving, formal verification, and the study of alternative logical systems.