From Cicero, The Nature of the Gods
We of the Academy are not people who will accept nothing as true. But we do hold that every true perception has in it an admixture of falsehood so similar to the truth that we have no certain criteria of judgment and assent. It follows that we can attain only to a number of probable truths, which although they cannot be proved as certainties, yet may appear so clear and convincing that a wise man may well adopt them as a rule of life.
But here is the problem: is the position that every true perception has in it an admixture of falsehood so similar to the truth that we have no certain criteria of judgment and assent itself afflicted with the difficulty attending upon every true perception? In other words, is Cicero’s point of view subject to the limitation applied by him to all points of view? If so, if every true perception has such an admixture of falsehood that it cannot be certainly, but only probably, judged and assented to, then this true perception also has in it an admixture of falsehood –and then we have arrived precisely nowhere. Nothing can be certainly known to be true, but only probable: but it is only probable, not certain, that nothing can be certainly, but only probably, known to be true. The only way out of the dilemma seems to be that this is not a true perception –but in that case we can stop worrying about it. Apart from its self-undercutting nature, it is a hard position to maintain; earlier Cicero had stated: “But in this medley of conflicting opinions, one thing is certain. Though it is possible that they are all of them false, it is impossible that more than one of them is true.” Again, is this true perception only probably distinguishable from error? If so, how is it then certain? But if not, how is it then that every true perception has in it such an admixture of falsehood so similar to the truth that we have no certain criteria of judgment and assent?