Sextus Empiricus Pyrrhoniae Hypotyposes 2.13.182

[182] Since, then, proof is non-evident, owing to the controversy which exists concerning it (for things controverted, in so far as controverted, are non- evident), its existence is not self-evident but needs to be established for us by proof. The proof, then, by which proof is established will not be evident and agreed (for we are now inquiring whether proof in general exists), and being thus in dispute and non- evident it will need another proof, and this again a third, and so on ad infinitum. But it is impossible to prove an infinite series; therefore it is impossible to show that proof exists.