Sextus Empiricus Pyrrhoniae Hypotyposes 1.14.61

[61] It is, indeed, absurd to attempt to establish the matter in question by means of the matter in question, ¹ since in that case the same thing will be at once believed and disbelieved,—believed in so far as it purports to prove, but disbelieved in so far as it requires proof,—which is impossible. Consequently we shall not possess a proof which enables us to give our own sense-impressions the preference over those of the so-called irrational animals. If, then, owing to the variety in animals their sense-impressions differ, and it is impossible to judge between them, we must necessarily suspend judgement regarding the external underlying objects.