Mathematicians prove theorems in a semi-formal setting, providing what we’ll call informal proofs. There are various philosophical reasons not to reduce informal provability to formal provability within some appropriate axiomatic theory (Leitgeb, 2009; Marfori, 2010; Tanswell, 2015), but the main worry is that we seem committed to all instances of the so-called reflection schema: B(φ) → φ (where B stands for the informal provability predicate). Yet, adding all its instances to any theory for which Löb’s theorem for B holds leads to inconsistency.Currently existing approaches (Shapiro, 1985; Horsten, 1996, 1998) to formalizing the properties of informal provability avoid contradiction at a rather high price. They either drop one of the Hilbert-Bernays conditions for the provability predicate, or use a provability operator that cannot consistently be treated as a predicate.Inspired by (Kripke, 1975), we investigate the strategy which changes the underlying logic and treats informal provability as a partial notion. We use non-deterministic matrices to develop a three-valued logic of informal provability, which avoids some of the above mentioned problems.

In this paper, a modular approach for non-deterministic semantics for (non-normal) modal logics is developed. In particular, our aim is to improve and reinterpret some results from Omori and Skurt (2016, IfCoLog J. Logics Appl., 3, 815–845) and Coniglio et al. (2015, J. Appl. Non-Class. Log., 25, 20–45) regarding modal systems T, TB, S4 and S5. More economical axiomatizations make the rule of necessitation modular, thus providing non-deterministic semantics for (NEC)-free fragments for all the investigated systems. Moreover, by fixing the interpretation of all connectives but the modal ones, a combinatorial outlook at their matrices is provided to the effect that a new modal system and simplification of those for T and S4 are achieved.

The main goal of this paper is to provide an abstract framework for constructing proof systems for various many-valued logics. Using the framework it is possible to generate strongly complete proof systems with respect to any finitely valued deterministic and non-deterministic logic. I provide a couple of examples of proof systems for well-known many-valued logics and prove the completeness of proof systems generated by the framework.

Classical logic of formal provability includes Löb’s theorem, but not reflection. In contrast, intuitions about the inferential behavior of informal provability (in informal mathematics) seem to invalidate Löb’s theorem and validate reflection (after all, the intuition is, whatever mathematicians prove holds!). We employ a non-deterministic many-valued semantics and develop a modal logic T-BAT of an informal provability operator, which indeed does validate reflection and invalidates Löb’s theorem. We study its properties and its relation to known provability-related paradoxical arguments. We also argue that T-BAT is a fairly sensible candidate for a formal logic of informal provability.