Plato, in his mode of communicating instruction, and promulgating his opinions, had not strictly adhered to the method of his master Socrates. He held the concurrence of memory, with a recent impression, to be a criterion of truth, and he taught that opinions might be formed from the comparison of a present with a recollected perception. But his successors, both in the Academy and Lyceum, departed from the Socratic method still more widely. They renounced the maxim, of affirming nothing; and instead of explaining everything with a doubting reserve, they converted philosophy, as it were, into an art, and formed a system of opinions, which they delivered to their disciples as the peculiar tenets of their sect. They inculcated the belief, that our knowledge has its origin in the senses—that the senses themselves do not judge of truth, but the mind through them beholds things as they really are—that is, it perceives the ideas which always subsist in the same state, without change; so that the senses, through the medium of the mind, may be relied on for the ascertainment of truth. Such was the state of opinions and instruction in the Academy when Arcesilaus, who was the sixth master of that school from Plato, and in his youth had heard the lessons of Pyrrho the sceptic, resolved to reform the dogmatic system into which his predecessors had fallen, and to restore, as he conceived, in all its purity, the Socratic system of affirming nothing with certainty. This founder of the New, or Middle Academy as it is sometimes called, denied even the certain truth of the proposition that we know nothing, which Socrates had reserved as an exception to his general principle. While admitting that there is an actual certainty in the nature of things, he rejected the evidence both of the senses and reason as positive testimony; and as he denied that there existed any infallible criterion of truth or falsehood, he maintained that no wise man ought to give any proposition whatever the sanction of his assent. He differed from the Sceptics or Pyrrhonists only in this, that he admitted degrees of probability, whereas the Sceptics fluctuated in total uncertainty.