Let us consider the first of these causes. Aenesidemus, although not the first of the later Sceptics, was apparently the first to separate himself from the Academy. He was the founder of a new movement, the attempt to revive the older Scepticism as taught by Pyrrho and Timon, and separate it from the dogmatic teachings of the Stoics which were so greatly affecting the Scepticism of the New Academy. It was the spirit of his time to seek to sustain all philosophical teaching by the authority of as many as possible of the older philosophers, and he could hardly escape the tendency which his training in the Academy had unconsciously given him. Therefore we find him trying to prove that the philosophy of Heraclitus follows from Scepticism. It is not necessary either to explain the matter, as both Hirzel and Natorp so ingeniously attempt to do, by claiming that the truth of contradictory predicates which Aenesidemus accepted from Heraclitus referred only to phenomena. The history of philosophy gives us abundant proof of the impossibility of absolute Scepticism, and Aenesidemus furnishes us with one example of many of this impossibility, and of the dogmatism that must exist in connection with all thought. In the case of Aenesidemus, who evidently gave the best efforts of his life to establish the Sceptical School, the dogmatism was probably unconscious. That he remained to the end a Sceptic is shown by the fact that he was known as such to posterity. Nowhere do we find a change of basis referred to in regard to him, and Sextus, in refuting the mistakes which he attributes to Aenesidemus, does it, as it were, to point out something of which Aenesidemus had been unconscious. Let us consider here the second cause of Aenesidemus' Dogmatism, the psychological effect upon himself of formulating Sceptical beliefs. The work that he did for the Sceptical School was a positive one. It occupied years of his life, and stamped itself upon his mental development. In formulating Scepticism, and in advocating it against the many enemies of the School, and amidst all the excitement of the disruption from the Academy, and of establishing a new School, it was inevitable that his mind should take a dogmatic tendency. He remained a Sceptic as he had always been, but must have grown dogmatic in his attitude towards the Sceptical formulae, and was thus able to adopt some of the teachings of Heraclitus, unconscious of their inconsistency.