The third leading Stoic after Zeno of Citium and Cleanthes, and possibly the most productive philosopher of all time, having written 705 books, none of which survive (however, ancient books were relatively short; see also Dewey). Chrysippus was originally a pupil of Arcesilaus, and was converted to Stoicism by Cleanthes. He enjoyed a considerable reputation as a logician, and there is some evidence that he commanded the fundamental idea of a truth function, which then lay dormant until the 20th century. He also held a cognitive theory of the emotions, which he thought consisted in judgements of the value of things.