This advanced study of systems of propositional logic offers a comprehensive account of a wide variety of logical systems and encourages students to take a critical stance toward the subject. A great variety of systems and subsystems are defined and compared as regards their deductive power and relation to their model theory. Special attention is given to the weakenings of classical logic and a more refined treatment of modal logic is presented.

This book examines an abstract mathematical theory, placing special emphasis on results applicable to formal logic. If a theory is especially abstract, it may find a natural home within several of the more familiar branches of mathematics. This is the case with the theory of closure spaces. It might be considered part of topology, lattice theory, universal algebra or, no doubt, one of several other branches of mathematics as well. In our development we have treated it, conceptually and methodologically, as part of topology, partly because we first thought ofthe basic structure involved...