Aenesidemus (Αἰνησίδημος or Αἰνεσίδημος) was a 1st-century BC Greek Pyrrhonist philosopher from Knossos who revived the doctrines of Pyrrho and introduced ten skeptical "modes" (tropai) for the suspension of judgment. He broke with the Academic Skepticism that was predominant in his time, synthesizing the teachings of Heraclitus and Timon of Phlius with philosophical skepticism. Although his primary work, the Pyrrhonian Discourses, has been lost, an outline of the work survives from the later Byzantine empire, and the description of the modes has been preserved by few ancient sources.
There is no definitive evidence about the life of Aenesidemus. What little we know is from a description of his Pyrrhonian Discourses in the Myriobiblion of Photius from the 9th century, as well as a few mentions in the works of Sextus Empiricus, and to a lesser extent by Diogenes Laërtius.
Whether Aenesidemus re-founded the Pyrrhonist school or merely revitalized it is unknown: while Diogenes claims an unbroken lineage of teachers of Pyrrhonism from Pyrrho through Sextus, with Aenesidemus' teacher being Heraclides of Tarentum, little is known about several of the names between Timon of Phlius and Aenesidemus, so this lineage is suspect. Photius says that Aenesidemus dedicated his Pyrrhonian Discourses to Lucius Aelius Tubero, a friend of Cicero and member of the Academy, whom Photius described as a colleague of Aenesidemus. Based on this information, scholars have assumed that Aenesidemus himself was also a member of the Academy. Furthermore, it has been assumed that he took part under the leadership of Philo of Larissa and probably adopted Pyrrhonism either in reaction to Antiochus of Ascalon's Middle Platonist introduction of Stoic and Peripatetic dogma into the Academy, or Philo's acceptance of provisional beliefs.
Aenesidemus' philosophy consisted of four main parts, the reasons for scepticism and doubt, the attack on causality and truth, a physical theory, and a theory of morality.