opposition, square of

Geometrical representation of the traditional logical relations of opposition (contradictories, contraries, subalterns) among the four types of propositions (universal affirmative or A-proposition, universal negative or E, particular affirmative or I, particular negative or O) in the Aristotelian doctrine of the syllogism.

At the top left is the A; at the top right is the E; below the A is the I; and below the E is the O. The A and the O, like the E and I, are contradictories (diagonal relations); the A and the E are contraries; and the I is subaltern to (hence implied by) the A, the O subaltern to the E. Whereas contradictories have opposite truth-values (one true, the other false), contraries cannot both be true but can both be false.

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▪ logic

      in traditional logic, a diagram exhibiting four forms of a categorical proposition (q.v.), or statement, with the same subject and predicate, together with their pairwise relationships:

      in which A, E, I, and O are of the forms “Every S is P,” “No S is P,” “Some S is P,” and “Some S is not P.” As shown on the square, “Every swan is white” is the contrary of “No swan is white” and the contradictory of “Some swans are not white.” Conclusions drawn from one of these forms to another (as in subalternation) are said to be obtained by immediate inference.

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