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Arcesilaus (Ἀρκεσίλαος) (c.316-c.241 BC) was a Greek philosopher and founder of the New, or Middle, Academy—the sceptical phase.

Born at Pitane in Aeolia, he was trained by Autolycus the mathematician and later at Athens by Theophrastus and Crantor, by whom he was led to join the Academy. He subsequently became intimate with Polemon and Crates, whom he succeeded as head of the school (σχολάρχης).

Diogenes Laërtius says that similarly to 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. His doctrines, which must be gathered from the writings of others (Cicero, Academica 1. 12, iv. 24; De Oratorio iii. 18; Diogenes Laërtius iv. 28; Adv. Math. vii. 150, Pyrrh. Hyp. i. 233; Sextus Empiricus Against the Logicians), present an attack on the Stoic notion of φαντασία καταληπτική (manifest presentation, or verifiable sense-impression) as self-warranting criterion of truth and are based on the sceptical element latent in the later writings of Plato.

He held that strength of intellectual conviction cannot be regarded as valid, as much as it is characteristic equally of contradictory convictions. The uncertainty of sensible 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 have to be content with an agnostic attitude. He made use of the Socratic method of instruction and left no writings. His arguments were marked by incisive humour and fertility of ideas.

See R. Brodeisen, De Arcesila philosopizo (1821); Aug. Geffers, Arcesila (1842); Ritter and Preller, Hist. philos. graec. (1898).

• Stanford Encyclopedia of Philosophy entry

This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.