Diogenes Laertius Vitae philosophorum 7.1.25

[25] According to Hippobotus he forgathered with Diodorus, with whom he worked hard at dialectic. And when he was already making progress, he would enter Polemo’s school: so far from all selfconceit was he. In consequence Polemo is said to have addressed him thus: You slip in, Zeno, by the garden door—I’m quite aware of it—you filch my doctrines and give them a Phoenician make-up. A dialectician once showed him seven logical forms concerned with the sophism known as The Reaper, and Zeno asked him how much he wanted for them. Being told a hundred drachmas, he promptly paid two hundred: to such lengths would he go in his love of learning. They say too that he first introduced the word Duty and wrote a treatise on the subject. It is said, moreover, that he corrected Hesiod’s lines thus: