  Given that not every apparent sentential negation is contradictory, is every contradictory negation sentential? Within propositional logic, contradictory negation is a self-annihilating operator: ¬(¬Φ) is equivalent to Φ. This is explicitly recognized in the proto-Fregean Stoic logic of Alexander of Aphrodisias: “‘Not: not: it is day’ differs from ‘it is day’ only in manner of speech” (Mates 1953: 126). The Stoics' apophatikon directly prefigures the iterating and self-cancelling propositional negation of Frege and Russell. As Frege puts it (1919: 130), “Wrapping up a thought in double negation does not alter its truth value.” The corresponding linguistic principle is expressed in the grammarians' bromide, “Duplex negatio affirmat.”