Sextus Empiricus Pyrrhoniae Hypotyposes 2.7.78

[78] Once again they will need a second presentation to judge the first, and a third to judge the second, and so on ad infinitum. But it is impossible to judge an infinite series; and therefore it is impossible to discover what sort of presentations we ought to employ as criteria, and what we ought not. Seeing, then, that, even should we grant that one ought to judge objects according to presentations, whether we adopt the alternative of trusting all as criteria or that of trusting some and distrusting others, in either case the argument is overthrown, and we are forced to conclude that we ought not to adopt presentations as criteria for the judging of objects.