In this paper I shall give a solution of the decision problem for the Lewis systems S2 and S4; i.e., I shall establish a constructive method for deciding whether an...

Classical logic requires each singular term to denote an object in the domain of quantification—which is usually understood as the set of “existing” objects. Free logic does not. Free logic...

Every consistent set of formulae has a maximal consistent superset. Each consistent theory is contained in a maximally consistent theory.