 (ii) The Criterion of Truth. Zeno, the founder of Stoicism, and roughly 20–30 years Arcesilaus’ senior, was for a time a student at the Academy. He was still in the Academy when he formulated key Stoic doctrines. Like Arcesilaus, he claims Socratic ancestry. Zeno is inspired by some of the same ideas that inspire the skeptics. In particular, he engages with the Socratic idea that knowledge is integral to virtue. Contrary to Arcesilaus, Zeno aims to give accounts of knowledge and virtue, and holds them up as ideals that human nature permits us to achieve. For him, knowledge is very difficult to attain, but ultimately within the reach of human beings. From the point of view of Arcesilaus, Zeno’s claim to Socratic heritage is almost offensive: Zeno seems to be too optimistic about our cognitive powers to be following Socrates (Frede 1983). Scholars have traditionally envisaged an exchange of arguments between Zeno and Arcesilaus, where each modified his views in the light of the other’s criticism. However, Zeno most likely formulated his views between 300–275, and Arcesilaus argued against him c. 275 to 240, when Zeno (who died c. 263) was probably already retired (Brittain 2006, xiii; Alesse 2000, 115 f.; Long 2006, ch. 5).