Sextus Empiricus Pyrrhoniae Hypotyposes 2.13.191

[191] And as the clause ‟proof exists not” is not in conflict with ‟proof exists,” the disjunctive ‟Either proof exists or proof exists not” will not be valid; for the valid disjunctive promises that one of its clauses is valid, but the other or others false and contradictory. Or else, if the disjunctive be valid, the hypothetical premiss ‟If proof exists not, proof exists” is, in turn, found to be fallacious, as composed of conflicting clauses. So then the premisses in the foregoing argument are discordant and mutually destructive; wherefore the argument is not valid.