Sextus Empiricus Pyrrhoniae Hypotyposes 2.13.183

[183] But neither can it be revealed by means of a sign. For since it is a matter of inquiry whether sign exists, ¹ and since the sign needs proof to ensure its reality, we find ourselves involved in circular reasoning— the proof requiring a sign, and the sign in turn a proof; which is absurd. And for these reasons neither is it possible to decide the controversy regarding proof, seeing that the decision requires a criterion, but—because it is a matter of inquiry, as we have shown, ² whether a criterion exists, and consequently the criterion needs a proof showing the existence of a criterion—we are again involved in the perplexity of circular reasoning.