Bertrand Russell, Herbrand’s theorem, and the assignment statement

Abstract

While propositional modal logic is a standard tool, first-order modal logic is not. Indeed, it is not generally understood that conventional first-order syntax is insufficiently expressible. In this paper we sketch a natural syntax and semantics for first-order modal logic, and show how it easily copes with well-known problems. And we provide formal tableau proof rules to go with the semantics, rules that are, at least in principle, automatable.