Sextus Empiricus Pyrrhoniae Hypotyposes 2.13.189

[189] If the hypothetical premiss ‟If proof exists, proof exists” is valid, the contradictory of its consequent, namely ‟proof does not exist,” must conflict with ‟proof exists,” for this is the antecedent of the hypothetical premiss. But, according to them, it is impossible for a hypothetical premiss to be valid when composed of conflicting clauses. For the hypothetical premiss promises that when its antecedent is true its consequent is also true, whereas conflicting clauses contrariwise promise that if either one of them is true the other cannot possibly be true. If therefore the premiss ‟If proof exists, proof exists” is valid, the premiss ‟If proof exists not, proof exists” cannot be valid.