Sextus Empiricus Pyrrhoniae Hypotyposes 2.14.202

[202] Whence we argue again that if for establishing the disjunctive proposition and the negative of the conjunctive we require to apprehend beforehand the fact that the judgements they contain are incapable of co-existence, while they believe that they are deducing this incapacity for co-existence by means of both the disjunctive and the negative conjunctive, we involve ourselves in circular reasoning, seeing that we are unable either to give credence to the aforesaid premisses without having apprehended the incapacity for co-existence of the judgements they contain, or to affirm positively that incapacity before concluding the syllogisms based on these premisses.