Arcesilaus (ärsĕs´Ĭlā´əs), c.316–c.241 BC, Greek philosopher of Pitane in Aeolis. He was the principal figure of the Middle Academy. Despite his position in the Academy, his teachings diverged from Platonic doctrine. By emphasizing the doubt expressed by Socrates as to the possibility of gaining knowledge, he took a position comparable to that of the Skeptics (see skepticism). He argued that knowledge and opinion could not be distinguished from each other, so that what anyone claims to know may be more or less probable but not certain. In denying the possibility of certainty he was a major opponent of the Stoics (see Stoicism). Arcesilaus indirectly influenced Carneades and his school.
See A. A. Long, The Hellenistic Philosophers (2 vol. 1987).