Arcesilaus (/ˌɑːrsɛsᵻˈleɪ.əs/; Greek: Ἀρκεσίλαος; 316/5–241/0 BC) was a Greek philosopher and founder of the Second or Middle Academy—the phase of Academic skepticism. Arcesilaus succeeded Crates as the sixth 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 skeptical phase of the Academy. His chief opponents were the Stoics and their belief that reality could be comprehended with certainty.
Arcesilaus was born in Pitane in Aeolis. His early education was provided by Autolycus the mathematician, with whom he migrated to Sardis. Afterwards, he studied rhetoric in Athens; but adopted philosophy and became a disciple first of Theophrastus and afterwards of Crantor. He subsequently became intimate with Polemo and Crates, and eventually became head of the school (σχολάρχης).
In Greek mythology, Arcesilaus was one of the Greek leaders in the Trojan War, from Boeotia, commander of ten ships. He was son of Areilycus and Theobule, and brother of Prothoenor. In one source though, he is called a son of Alector and Cleobule, and thus half-brother to Leitus and Clonius. He was killed by Hector.Leitus brought his remains back to Boeotia and buried them near Lebadea, on the banks of River Hercyna.
Arcesilaus (fl. 3rd century) was a Roman senator who was appointed consul in AD 267.
Possibly of Greek descent, Arcesilaus was probably the grandson of Titus Flavius Arcesilaus, who was a Flamen of the Arval Brethren, and who served as the magister creatus throughout the 220s. Arcesilaus himself was probably the Comes of Rome and Italia in AD 257. He was later made consul posterior alongside Ovinius Paternus in AD 267.