Sextus Empiricus Pyrrhoniae Hypotyposes 1.16.179

[179] And the reason why they do not allow that anything is apprehended through something else is this: If that through which an object is apprehended must always itself be apprehended through some other thing, one is involved in a process of circular reasoning or in regress ad infinitum. And if, on the other hand, one should choose to assume that the thing through which another object is apprehended is itself apprehended through itself, this is refuted by the fact that, for the reasons already stated, nothing is apprehended through itself. But as to how what conflicts with itself can possibly be apprehended either through itself or through some other thing we remain in doubt, so long as the criterion of truth or of apprehension is not apparent, and signs, even apart from demonstration, are rejected, as we shall discover in our next Book. ¹

For the present, however, it will suffice to have said thus much concerning the Modes leading to suspension of judgement.