Arcesilaus (//; Greek: Ἀρκεσίλαος; 316/5–241/0 BC) was a Greek Hellenistic philosopher. He was the founder of Academic Skepticism and what is variously called the Second or Middle or New Academy—the phase of the Academy in which it embraced philosophical skepticism.
Arcesilaus succeeded Crates of Athens as the sixth scholarch of the Academy around 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.
In Athens Arcesilaus interacted with the Pyrrhonist philosopher, Timon of Phlius, whose philosophy appears to have influenced Arcesilaus to become 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 opponent was his contemporary, Zeno of Citium, the founder of Stoicism, whose dogma of katalepsis (i.e., that reality could be comprehended with certainty) Arcesilaus denied.