The scepticism founded by Pyrrho soon became extinct, but an essentially similar doctrine began to be taught in the school of Plato. After the death of Plato, the Academy continued, under various leaders, to follow in the path marked out by the founder. But, under the leadership of Arcesilaus, scepticism was introduced into the school, and from that time, therefore, it is usually known as the New Academy, for though its historical continuity as a school was not broken, its essential character underwent change. What especially {365} characterized the New Academy was its fierce opposition to the Stoics, whom its members attacked as the chief dogmatists of the time. Dogmatism, for us, usually means making assertions without proper grounds. But since scepticism regards all assertions as equally ill-grounded, the holding of any positive opinion whatever is by it regarded as dogmatism. The Stoics were the most powerful, influential, and forceful of all those who at that time held any positive philosophical opinions. Hence they were singled out for attack by the New Academy as the greatest of dogmatists. Arcesilaus attacked especially their doctrine of the criterion of truth. The striking conviction which, according to the Stoics, accompanies truth, equally accompanies error. There is no criterion of truth, either in sense or in reason. "I am certain of nothing," said Arcesilaus; "I am not even certain that I am certain of nothing." But the Academics did not draw from their scepticism, as Pyrrho had done, the full logical conclusion as regards action. Men, they thought, must act. And, although certainty and knowledge are impossible, probability is a sufficient guide for action. Carneades is usually considered the greatest of the Academic Sceptics. Yet he added nothing essentially new to their conclusions. He appears, however, to have been a man of singularly acute and powerful mind, whose destructive criticism acted like a battering-ram not only upon Stoicism, but upon all established philosophies. As examples of his thoughts may be mentioned the two following. Firstly, nothing can ever be proved. For the conclusion must be proved by premises, which in turn require proof, and so _ad infinitum_. Secondly, {366} it is impossible to know whether our ideas of an object are true, i.e., whether they resemble the object, because we cannot compare our idea with the object itself. To do so would involve getting outside our own minds. We know nothing of the object except our idea of it, and therefore we cannot compare the original and the copy, since we can see only the copy. Later Scepticism.