The Skepticism of the Academy arose somewhat in this way. The rivalry of the Porch and the Older Academy had grown apace and had been a battle between two dogmatic Schools. The Academy was being worsted, its ancient spirit was waning, and it had gradually deserted speculation for ethics. Under Arcesilaus it was provoked to new life by the aggressive dogmatism of the Stoics. Speculation, which it had ignored, it now began to antagonize openly. Arcesilaus, in directing his attack against the doctrine of “apprehensive presentation” of the Stoics, came to conclusions but slightly different from Pyrrho. Carneades laid out for himself a twofold task: (1) to refute all existing dogmas, and (2) to evolve a theory of probability as the basis for practical activity. He applied his Skepticism not only to speculation, like Arcesilaus, but also to ethics and religion.[39] The Academy did not fully adopt Skepticism, but used it as a weapon against the Stoics. The Platonic tradition was kept alive within the School, and Skepticism made no advance in the Academy after Carneades. It did not even continue in the path marked out by him. In the next generation the Academy became eclectic. =3. The Third Period of Philosophic Skepticism――Sensationalistic Skepticism= (during two centuries or more of the Christian era). The chief representatives were Ænesidemus of Cnossus (first century A. D.), Agrippa (about 200 A. D.), and Sextus Empiricus (about 200 A. D.). This phase of Skepticism was represented mainly by physicians, with arguments based upon empirical physiological grounds. When the Academy passed from Skepticism to eclecticism, Skepticism became centred in Alexandria. For two centuries before Galen (131–201 A. D.) great discoveries had been made in medicine, but the meaning of the discoveries had not been apprehended. There was a general feeling among physicians of that time that there is no such thing as scientific certainty; and skeptical arguments were constructed; based on the empirical discoveries of the scientific circle of Alexandria. While the arguments of the Academy were mostly formal attacks against the Stoics, this Skeptical School of physicians returned to Pyrrhonism, immensely reinforced with scientific material. It strove in vain to disassociate itself from the Academy, for it used in one way or another the formal arguments of the Skeptics of the Academy. In his eight books on Pyrrhonism, Ænesidemus developed the reasons which induced Pyrrho to call in question the possibility of knowledge. These are known in philosophy as the ten “tropes,” or ten ways of justifying doubt.[40] They were badly arranged by Ænesidemus and reduced to five by Agrippa.[41]