Academic skepticism refers to the skeptical period of ancient Platonism dating from around 266 BC, when Arcesilaus became head of the Platonic Academy, until around 90 BC, when Antiochus of Ascalon rejected skepticism. Like their fellow Pyrrhonists, they maintained that knowledge of things is impossible. Ideas or notions are never true; nevertheless, there are degrees of probability, and hence degrees of belief, which allow one to act.