While the schools of Zeno and Epicurus seated at Athens were powerfully influencing Grecian thought, the former especially drawing to it the stronger and more thinking minds, resistance arose to them both in the chair of Plato. First Arcesilaus and then Carneades, who had succeeded to this office, set up in the middle Academy the school of Scepticism. While Stoics and Epicureans alike sought peace of mind through knowledge of the world and its laws, they on the contrary maintained that this same peace of mind could only be attained by renouncing all such knowledge.(437) They held that no truth and no certainty were given to man by the representations of his senses, by his feelings, and by his consciousness of these, which do not enable him to know the real being of anything.(438) Those who held this view would not say downright that what they contradicted was untrue: they were of opinion that it might be true, only there was no certitude of this, and therefore it must be left undetermined. The uncertainty was as great on the one side as on the other. Sextus Empiricus defined the state of skepsis to be "skilfulness in so setting forth appearances and reflections against each other, as to be brought through the equilibrium of opposing facts and grounds in their favour first to a suspension of judgment, and then to imperturbable tranquillity."
Carneades, whose life occupied the greater part of the second century before Christ, and who is extolled by Cicero as the keenest and most copious of disputants, was the man in whom this school of thought reached its highest point. He had appeared at Rome among a deputation of philosophers in the year 155, when his eloquence and earnestness made a great impression on his Roman hearers. This scepticism of the younger Academy however ran in accordance with the direction which the collective philosophy of the Greeks naturally took, and was carried out with an acuteness and a scientific ability which makes us recognise in it an important member of philosophical development.(439) Carneades subjected the stoic doctrine as to God in particular to a criticism the range of which went far beyond the dogmas of this school, and in fact tended to represent every conviction as to the existence of the godhead, and every religious belief, as something impossible and untenable.(440) This, however, as Cicero repeatedly assures us, was not done for the purpose of destroying belief in the gods, but only to point out the weakness and groundlessness of stoic doctrines. It is chiefly in his assaults on the assertions and assumptions of his adversaries that Carneades is victorious: when he attempts anything positive on his own side, it amounts to this, that a rational man will take probability for his guide, when he cannot be assured of truth: and his chief merit appears to have been in more accurately determining the degrees of probability.(441)