Sextus Empiricus Pyrrhoniae Hypotyposes 2.9.85

[85] Seeing, then, that there is a controversy amongst the Dogmatists regarding ‟the true,” since some assert that something true exists, others that nothing true exists, it is impossible to decide the controversy, because the man who says that something true exists will not be believed without proof, on account of the controversy; and if he wishes to offer proof, he will be disbelieved if he acknowledges that his proof is false, whereas if he declares that his proof is true he becomes involved in circular reasoning and will be required to show proof of the real truth of his proof, and another proof of that proof, and so on ad infinitum. But it is impossible to prove an infinite series; and so it is impossible also to get to know that something true exists.