For it would happen, firstly, if any one were to postulate opposites, affirmation and negation; secondly, if he were to postulate the contrary terms of an antithesis, eg that the same thing is good and evil; thirdly, suppose anyone were to claim something universally and then proceed to postulate its contradictory in some particular case, eg if having assumed that the knowledge of contraries is one, he were to claim that the knowledge of what makes for health or for disease is different; or suppose him, after postulating the latter view, to try to secure universally the contradictory statement.