Academic skepticism refers to the skeptical period of the Academy dating from around 266 BCE, when Arcesilaus became scholarch, until around 90 BCE, when Antiochus of Ascalon rejected skepticism, although individual philosophers, such as Favorinus and his teacher Plutarch, continued to defend skepticism after this date. Unlike the existing school of skepticism, the Pyrrhonists, they maintained that knowledge of things is impossible. Ideas or notions are never true; nevertheless, there are degrees of plausibility, and hence degrees of belief, which allow one to act. The school was characterized by its attacks on the Stoics, particularly their dogma that convincing impressions led to true knowledge. The most important Academics were Arcesilaus, Carneades, and Philo of Larissa. The most extensive ancient source of information about Academic skepticism is Academica, written by the Academic skeptic philosopher Cicero.
Overview
Greek philosophical skepticism, as a distinct philosophical movement, began with Pyrrho of Elis (), with antecedents in Xenophanes and Democritus. His followers, the Pyrrhonists, pointed out the problem of the criterion: that our theories and our sense impressions are unable to accurately distinguish truth from falsehood; therefore we must suspend judgment (epoche). They were consistent enough to extend their doubt even to their own principle of doubt, making their skepticism universal, thus escaping reproach for basing it upon a fresh dogmatism. Mental imperturbability (ataraxia) was the result to be attained by cultivating such a frame of mind.
Around 266 BCE, Arcesilaus became head of the Platonic Academy. He adopted skepticism as a central tenet of Platonism, making Platonism nearly the same as Pyrrhonism. After Arcesilaus, the Academics diverged from Pyrrhonism. 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. Following the death of the Pyrrhonist Timon of Phlius, the Platonic Academy became the primary advocate of skepticism until the mid-first century BCE. While early Academic skepticism was influenced in part by Pyrrho, it grew more and more dogmatic until Aenesidemus, in the first century BCE, broke with the Academic skeptics and adopted Pyrrhonism, denouncing the Academy as "Stoics fighting against Stoics."
The Academics did not doubt the existence of truth; they just doubted that humans had the capacities for obtaining it. They based this position on Plato's Phaedo, in which Socrates discusses how knowledge is not accessible to mortals.
While the objective of the Pyrrhonists was the attainment of ataraxia, after Arcesilaus the Academics did not hold up ataraxia as the central objective. The Academics focused on criticizing the dogmas of other schools of philosophy, in particular of the dogmatism of the Stoics. They acknowledged some vestiges of a moral law within, at best but a plausible guide, the possession of which, however, formed the real distinction between the sage and the fool. Slight as the difference may appear between the positions of the Academics and the Pyrrhonists, a comparison of their lives leads to the conclusion that a practical philosophical moderation was the characteristic of the Academics whereas the objectives of the Pyrrhonists were more psychological. The second-century Roman historian Aulus Gellius described the distinction between the Academic skeptics and the Pyrrhonists as follows:
"...the Academics apprehend (in some sense) the very fact that nothing can be apprehended, and they determine (in some sense) that nothing can be determined, whereas the Pyrrhonists assert that not even that seems to be true, since nothing seems to be true."
Arcesilaus
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 and denied the possibility of even the Socratic minimum of certainty: "I cannot know even whether I know or not."