Modal logic for reasoning about uncertainty and confusion (2505.02548v2)

Abstract: We consider a modal logic that can formalise statements about uncertainty and beliefs such as I think that my wallet is in the drawer rather than elsewhere' orI am confused whether my appointment is on Monday or Tuesday'. To do that, we expand G\"{o}del modal logic $\mathbf{K}\mathsf{G}$ with the involutive negation $\sim$ defined as $v({\sim}\phi,w)=1-v(\phi,w)$. We provide semantics with the finite model property for our new logic that we call $\mathbf{K}\mathsf{G_{inv}}$ and show its equivalence to the standard semantics over $[0,1]$-valued Kripke models. Namely, we show that $\phi$ is valid in the standard semantics of $\mathbf{K}\mathsf{G_{inv}}$ iff it is valid in the new semantics. Using this new semantics, we construct a constraint tableaux calculus for $\mathbf{K}\mathsf{G_{inv}}$ that allows for an explicit extraction of countermodels from complete open branches and then employ the tableaux calculus to obtain the PSpace-completeness of the validity in $\mathbf{K}\mathsf{G_{inv}}$.