Selcuk Kaan Tabakci

Carnap’s (Categoricity) Problem concerns the relationship between (rules of) inference and model-theoretic values. In particular, it asks whether proof-theoretic constraints are ‘strong enough’ to uniquely determine intended semantic values. Carnap [20] demonstrated that already in the classical bivalent setting this is not the case for the majority of the usual logical constants. To remedy this underdetermination of ‘semantics by syntax’ a variety of solution strategies has been explored in the literature. This article is a philosophical-logical survey of these attempts, comparing them with respect to scope, motivation, and success. Besides the mathematical interest held by Carnap’s Problem, the underdetermination it uncovers has significant consequences for a variety of philosophical projects and positions, warranting a systematic study of attempts at resolving it.

In this paper, I argue against the thesis that the meaning of ‘computability’ is logic-dependent. I do this from a category-theoretic perspective. Applying a method due to Mortensen and Lavers [26], I show that we can dualize the internal logic of the effective topos, in order to obtain a model of paraconsistent computability theory. Since the dualization leaves the structural properties of universal constructions in the topos unchanged, in particular the properties of the natural numbers object, I conclude that, at least in this case, changing the logic does not change our characterization of computability.