Aenesidemus (1st century BCE) was a Greek sceptical philosopher, born in Knossos on the island of Crete. Very little was known about Aenesidemus’s life. He taught in Alexandria and flourished shortly after the life of Cicero and was associated with the Athenian Academy around the time of its collapse in 87 BCE. He was party to the dispute between Philo of Larissa, who advocated a mild form of skepticism in the form of an externalist, coherentist epistemology, and Antiochus of Ascalon, whose epistemology was basically that of Stoic foundationalism.

The Academy had been for two centuries the home of epistemological skepticism, directed largely against the optimistic epistemology of the Stoics, who posited “apprehensive impressions” (phantasiai katalêptikai), which carried their own guarantee of truth. Aenesidemus saw Philo and Antiochus as betraying that heritage, as “Stoics fighting with Stoics” (Photius, Library Catalogue 212), and resolved to “philosophize after the fashion of Pyrrho.”

Aenesidemus wrote eight books of Pyrrhonian Discourses, which Photius summarized: “the whole aim of the book is to ground the view that there is no ground for apprehension, whether through perception or thought.” The main burden of the Discourses, Photius says, is to establish that nobody really grasps anything. However, only Pyrrhonian skeptics are aware of this ignorance, while everyone else falsely considers themselves to be in possession of secure knowledge. This false conviction, and the inevitable disputes that follow from the evident fact that different people hold different and incompatible beliefs, leads the Dogmatists (“belief-holders,” as skeptics styled their opponents) into “ceaseless torments.”

Skeptics, having no beliefs, avoid these torments; indeed they “are happy … in the wisdom of knowing that they have firm apprehension of nothing.”