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Academic skepticism refers to the skeptical period of ancient Platonism dating from around 266 BC, when Arcesilaus became head of the Platonic Academy, until around 90 BC, when Antiochus of Ascalon rejected skepticism. Like their fellow Pyrrhonists, they maintained that knowledge of things is impossible. Ideas or notions are never true; nevertheless, there are degrees of probability, and hence degrees of belief, which allow one to act. The school was characterized by its attacks on the Stoics and on their belief in convincing impressions which lead to true knowledge. The most important Academic skeptics were Arcesilaus, Carneades, and Philo of Larissa.
Greek skepticism, as a distinct school, began with Pyrrho of Elis (c. 360 BC-c. 270 BC), about whom very little is known. His followers, the Pyrrhonists, maintained that knowledge of things is impossible and that we must assume an attitude of reserve (epoche). They were consistent enough to extend their doubt even to their own principle of doubt. They thus attempted to make their skepticism universal, and to escape the reproach of basing it upon a fresh dogmatism. Mental imperturbability (ataraxia) was the result to be attained by cultivating such a frame of mind.
Around 266 BC, Arcesilaus became head of the Platonic Academy, and adopted skepticism as a central tenet of Platonism. This skeptical period of ancient Platonism, from Arcesilaus to Philo of Larissa, became known as the New Academy, although some ancient authors added further subdivisions, such as a Middle Academy. The Academic skeptics do not seem to have doubted the existence of truth in itself, only the capacities for obtaining it. They differed from the principles of the Pyrrhonists in the practical tendency of their doctrines: while the object of the Pyrrhonists was the attainment of perfect equanimity, it seems that the Academics were less overborne than the Pyrrhonists by the practical issue of their doubts. The attitude maintained by the Academics contained a negative criticism of the views of others, in particular of the dogmatism of the Stoics. But they acknowledged some vestiges of a moral law within, at best but a probable guide, the possession of which, however, formed the real distinction between the sage and the fool. Slight as the difference may appear between the speculative statements of the two schools, a comparison of their lives leads to the conclusion that a practical moderation was the characteristic of the Academic skeptics.
Up to Arcesilaus, the Platonic Academy accepted the principle of finding a general unity in all things, by the aid of which a principle of certainty might be found. Arcesilaus, however, broke new ground by attacking the very possibility of certainty. Socrates had said, "This alone I know, that I know nothing." But Arcesilaus went farther and denied the possibility of even the Socratic minimum of certainty: "I cannot know even whether I know or not."
The doctrines of Arcesilaus, which must be gathered from the writings of others, represent an attack on the Stoic phantasia kataleptike (Criterion) and are based on the skepticism which was latent in the later writings of Plato. Arcesilaus held that strength of intellectual conviction cannot be regarded as valid, inasmuch as it is characteristic equally of contradictory convictions. The uncertainty of sense data applies equally to the conclusions of reason, and therefore man must be content with probability which is sufficient as a practical guide. "We know nothing, not even our ignorance"; therefore the wise man will be content with an agnostic attitude.
The next stage in Academic skepticism was the moderate skepticism of Carneades, which he said owed its existence to his opposition to Chrysippus, the Stoic. Carneades is the most important of the Academic skeptics.
To the Stoic theory of perception, the phantasia kataleptike, by which they expressed a conviction of certainty arising from impressions so strong as to amount to science, he proposed the doctrine of acatalepsia, which denied any necessary correspondence between perceptions and the objects perceived. All our sensations are relative, and acquaint us, not with things as they are, but only with the impressions that things produce upon us. Experience, he said, clearly shows that there is no true impression. There is no notion that may not deceive us; it is impossible to distinguish between false and true impressions; therefore the Stoic phantasia kataleptike must be given up. There is no criterion of truth. Carneades also assailed Stoic theology and physics. In answer to the doctrine of final cause, of design in nature, he pointed to those things which cause destruction and danger to man, to the evil committed by men endowed with reason, to the miserable condition of humanity, and to the misfortunes that assail the good man. There is, he concluded, no evidence for the doctrine of a divine superintending providence. Even if there were orderly connexion of parts in the universe, this may have resulted quite naturally. No proof can be advanced to show that this world is anything but the product of natural forces.
Knowledge being impossible, a wise man should practise epoche (suspension of judgment). He will not even be sure that he can be sure of nothing. He saved himself, however, from absolute skepticism by the doctrine of probability, which may serve as a practical guide in life. Ideas or notions are never true, but only probable; nevertheless, there are degrees of probability, and hence degrees of belief, leading to action. According to Carneades, an impression may be probable in itself; probable and uncontradicted (not distracted by synchronous sensations, but shown to be in harmony with them) when compared with others; probable, uncontradicted, and thoroughly investigated and confirmed. In the first degree there is a strong persuasion of the propriety of the impression made; the second and third degrees are produced by comparisons of the impression with others associated with it, and an analysis of itself. Carneades left no written works; his opinions seem to have been systematized by his pupil Clitomachus, whose works, which included one "on suspension of judgment," were made use of by Cicero.
Philo of Larissa 
In Philo of Larissa we find a tendency not only to reconcile the internal divergences of the Academy itself, but also to connect it with parallel systems of thought. In general, his philosophy was a reaction against the skeptic or agnostic position of the Middle and New Academy in favour of the dogmatism of Plato. Philo of Larissa endeavoured to show that Carneades was not opposed to Plato, and further that the apparent antagonism between Platonism and Stoicism was because they were arguing from different points of view. From this syncretism emerged the eclectic Middle Platonism of Antiochus of Ascalon, the last product of Academic development.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Scepticism". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- This article incorporates text from a publication now in the public domain: Smith, William, ed. (1867). "Arcesilaus". Dictionary of Greek and Roman Biography and Mythology.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Academy, Greek". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Cicero, Acad. i. 12, iv. 24; De Orat. iii. 18; Diogenes Laertius iv. 28; Sextus Empiricus, Adv. Math. vii. 150, Pyrrh. Hyp. i. 233
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Arcesilaus". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Carneades". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Clitomachus". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Philo of Larissa". Encyclopædia Britannica (11th ed.). Cambridge University Press.
- Ancient Skepticism entry by Leo Groarke in the Stanford Encyclopedia of Philosophy
- Ancient Greek Skepticism entry in the Internet Encyclopedia of Philosophy by Harold Thorsrud