The twentieth century, and especially its second half, was marked by a fairly spectacular proliferation of what are sometimes called nonclassical logics. To understand this, one needs to see the matter in its historical context. There have been three great periods in the history of European logic: ancient Greece, medieval Europe, and, starting toward the end of the nineteenth century, the current period. Each period has been marked by the production of novel theories of the nature and extent of logical validity. Thus, in the ancient period, Aristotle, the Megarian, and the Stoic logicians offered different accounts of validity, the conditional, and modality. The medieval period tried to reconcile some of the differences in their heritage, and in the process produced numerous different accounts of the nature of the connectives, consequence, and supposition. Not surprisingly, in both periods there was...