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. Unlike the existing school of skepticism, the 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. The school was characterized by its attacks on the Stoics and on their belief in convincing impressions which lead to true knowledge. The most important Academic skeptics were Arcesilaus, Carneades, and Philo of Larissa.