  Aristotle distinguishes between two types of sentential opposition: contraries and contradictories. A contradictory pair of sentences (an antiphasis) consists of an affirmation and its negation (i.e. the negation that negates of the subject what the affirmation affirms of it). Aristotle assumes that—normally—one of these must be true, the other false. Contrary sentences are such that they cannot both be true. The contradictory of a universal affirmative is the corresponding particular negative; that of the universal negative the corresponding particular affirmative. A universal affirmative and its corresponding universal negative are contraries. Aristotle thus has captured the basic logical relations between monadic quantifiers (Int. 7).