From Knowledge to Conjectures: A Modal Framework for Reasoning about Hypotheses

This paper introduces a new family of cognitive modal logics designed to formalize conjectural reasoning: modal systems in which cognitive contexts extend known facts with hypothetical assumptions in order to explore their consequences. Unlike traditional doxastic and epistemic systems, conjectural logics rely on a principle, called Axiom C (varphi rightarrow Boxvarphi), through which established facts are preserved across conjectural layers. While Axiom C has often been treated with suspicion because of its association with modal collapse, we show that collapse does not arise from C alone, but requires either the presence of Axiom T or a concretely bivalent base logic. Accordingly, we avoid T and adopt a non-bivalent semantic framework, such as supervaluation-style semantics, Weak Kleene logic, or Description Logic, in which undefined propositions may coexist with modal assertions. This prevents modal collapse and preserves a distinction between factual and conjectural statements. Within this framework we define the modal systems KC and KDC, show that Axiom C directly implies 4 and 5, and prove that these systems are non-trivial, sound, and complete. An inclusion theorem links reality, doxastic states, epistemic states, and conjectural states via set-theoretic inclusion among valuations, providing a unified account of how these layers relate. Finally, we introduce a dynamic operator, mathsfsettle(p), which formalizes the transition by which a conjectural extension becomes designated reality, thereby motivating a corresponding Conjectural Dynamic Logic.