ABSTRACT
The earliest system of logic which has survived was formulated by Aristotle in the fourth century b.c., and for most of the last two thousand years it has been considered authoritative and in need of no more than minor modifications. Inference, for traditional logic, is either ‘immediate’ or ‘mediate’. A syllogism is an argument with two premisses and a conclusion; each of the three propositions which constitute the premisses and conclusion are of one of the four forms, A, E, I or O; the argument contains three ‘terms’. Although the doctrine of distribution is neither clear nor philosophically unobjectionable, it is convenient to retain it in an elementary exposition of formal logic. To reject it would necessitate the reformulation of much of traditional doctrine. The formal relations of propositions with identical terms of the four forms, A, E, I and O, were represented by traditional logicians by a diagram called the square of opposition.
