See A. A. Long, The Hellenistic Philosophers (2 vol. 1987).
Arcesilaus (Ἀρκεσίλαος) (ca. 316-ca. 241 BC) was a Greek philosopher and founder of the Second or Middle Academy—the skeptical phase of the Academy. Arcesilaus succeeded Crates as head (scholarch) of the Academy c. 264 BC. He did not preserve his thoughts in writing, so his opinions can only be gleaned second-hand from what is preserved by later writers. He was the first Academic to adopt a position of philosophical skepticism, that is, he doubted the ability of the senses to discover truth about the world, although he may have continued to believe in the existence of truth itself. This brought in the skepical phase of the Academy. His chief opponents were the Stoics and their belief that reality could be comprehended with certainty.
Diogenes Laërtius says that, like his successor Lacydes, he died of excessive drinking, but the testimony of others (e.g. Cleanthes) and his own precepts discredit the story, and he is known to have been much respected by the Athenians.
On the one hand, he is said to have restored the doctrines of Plato in an incorrupted form; while, on the other hand, according to Cicero, he summed up his opinions in the formula, "that he knew nothing, not even his own ignorance." There are two ways of reconciling the difficulty: either we may suppose him to have thrown out such aphorisms as an exercise for his pupils, as Sextus Empiricus, who calls him a Sceptic, would have us believe; or he may have really doubted the esoteric meaning of Plato, and have supposed himself to have been stripping his works of the figments of the Dogmatists, while he was in fact taking from them all certain principles.
The Stoics were the chief opponents of Arcesilaus; he attacked their doctrine of a convincing conception (katalêptikê phantasia) as understood to be a mean between science and opinion - a mean which he asserted could not exist, and was merely the interpolation of a name. It involved a contradiction in terms, as the very idea of phantasia implied the possibility of false as well as true conceptions of the same object.
It is a question of some importance as to how the skepticism of the Middle and New Academy was distinguished from that of Pyrrhonism. Admitting the formula of Arcesilaus, "that he knew nothing, not even his own ignorance," to be an exposition of his real sentiments, it was impossible in one sense that skepticism could proceed further: but the Academic skeptics do not seem to have doubted the existence of truth in itself, only our capacities for obtaining it. It differed also from the principles of the pure skeptic in the practical tendency of its doctrines: while the object of the one was the attainment of perfect equanimity, the other seems rather to have retired from the barren field of speculation to practical life, and to have 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 the lives of their founders and their respective successors leads to the conclusion, that a practical moderation was the characteristic of the Academic skeptics.