 Aenesidemus (first century BCE) was discontented with the views discussed in the Academy at his time, of which he began as an adherent. Philo’s proto-externalism as well as a counterposition formulated by Antiochus both appeared to him dogmatic. Aenesidemus aimed to revive a more radical skepticism, and left the Academy for this purpose. Arguably, he is the first Pyrrhonian skeptic. Aenesidemus wrote a treatise, the Pyrrhonian Discourses, probably similar in structure to Sextus’ Outlines of Pyrrhonism and partially preserved in a summary by Photius: a general account of skepticism, followed by books on particular philosophical questions (Hankinson 2010). The basic elements of Aenesidemus’ skepticism are: the skeptic puts appearances and thoughts into opposition; this generates equipollence (isostheneia, lit. “of equal weight”) between several appearances and/or thoughts; suspension of judgment follows; with it comes tranquility; and the skeptic leads a life according to appearances (DL 9.62, 78, 106–7). However, we do not know much detail of his views on these matters. Instead, Aenesidemus is famous for having developed Ten Modes or Tropes—forms of argument by which the sceptic puts appearances and thoughts into opposition. Key questions about Aenesidemus’ skepticism concern (i) the interpretation of his Modes, (ii) the relationship of his philosophy to competing theories, (iii) the scope of the Ten Modes, and (iv) the skeptic’s mode of speech.