Square of opposition

Square of opposition, 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...