Sextus Empiricus Pyrrhoniae Hypotyposes 2.5.34

[34] But even supposing we grant that Man is apprehended, it would not, probably, be possible to show that objects ought to be judged by him. For he who asserts that objects ought to be judged by Man will be asserting this either without proof or with proof. ¹ Not with proof; for the proof must be true and tested, and therefore tested by some standard. Since, then, we are unable to make an agreed statement as to the standard by which the proof itself can be tested (for we are still inquiring about the criterion ‟By whom”), we shall be unable to pronounce judgement on the proof, and therefore also to prove the criterion, which is the subject of discussion.