ion, custom, and
law, but without any belief in the essential validity or truth of
these criteria.

The New Academy.

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 resem