“ This is the statement of the law given by Aristotle (τὸ γὰρ αὐτὸ ὑπάρχειν τε καὶ μὴ ὑπάρχειν ἀδύνατον τῷ αὐτῷ καὶ κατὰ τὸ αὐτό, Metaph. Γ 3, 1005 b 19) . It takes no account of the truth of either proposition; if one is true, the other is not; one of the two must be true.
Modern logicians, following Leibnitz and Kant, have generally adopted a different statement, by which the law assumes an essentially different meaning. Their formula is “A is not not-A”; in other words it is impossible to predicate of a thing a quality which is its contradictory. ”