Who was Chrysippus, the third head of the Stoa?

Chrysippus of Soli (c. 279-206 BC) was the third head of the Stoic school, succeeding Cleanthes, who had succeeded the founder Zeno of Citium. Diogenes Laertius, the main ancient source for his life, credits Chrysippus with an extraordinary output - reportedly more than 700 treatises - though almost none survive complete; what remains is fragments quoted or summarized by later writers. His central contribution was turning Zeno's founding insights and Cleanthes's development of them into a rigorously argued logical and philosophical system capable of competing with the established schools of the Academy and the Lyceum. Diogenes Laertius preserves a line summing up how essential this work was seen to be: "If Chrysippus had not existed, neither would the Stoa" - meaning that without Chrysippus's systematic defense of Stoic doctrine, the school might well have faded rather than enduring for centuries. He was especially known for his work in logic, developing propositional arguments and forms of syllogism distinct from Aristotle's, and for defending Stoic determinism - the view that all events are bound up in a chain of causes - against the charge that this removed room for human responsibility, an argument he addressed through his idea of "co-fated" events, where an outcome and the effort that produces it are both fixed together. Diogenes Laertius also records a famously strange account of his death: at around age seventy, supposedly after drinking undiluted wine and watching a donkey eat figs, Chrysippus is said to have died laughing after joking that the donkey should be given wine to wash the figs down. Whatever the truth of that anecdote, his systematizing work is why the later, better-known Roman Stoics - Seneca, Epictetus, Marcus Aurelius - inherited a coherent philosophical structure rather than a founder's scattered teachings.