Sextus Empiricus Pyrrhoniae Hypotyposes 2.5.36

[36] Yet if they say that a man is to be the judge, that will be assuming the point at issue; ¹ while if they make another animal the judge, in what way do they come to adopt that animal for the purpose of judging whether Man is the criterion? If they do so without a judgement, it will not be believed, and if with a judgement, it in turn needs to be judged by something. If, then, it is judged by itself, the same absurdity remains (for the object of inquiry will be judged by the object of inquiry); and if by Man, circular reasoning is introduced; and if by some judge other than these two, we shall once again in his case demand the criterion ‟By whom,” and so on ad infinitum. Consequently we shall not be in a position to declare that objects ought to be judged by Man.