Chrysippus

Chrysippus of Soli was the third head of the Stoa and so formidable a systematiser that the ancients said, 'without Chrysippus there would be no Stoa.' In some seven hundred works (almost all now lost) he gave the school the rigorous form it would keep for centuries, defending and elaborating its physics, ethics, and above all its logic.

He was the great logician of antiquity after Aristotle, developing a propositional logic of conditionals and inference that anticipated much later thought, and working out the Stoic account of fate, providence, and the compatibility of destiny with human responsibility.