Chrysippus and the Stoic Revolution in Logic

While Aristotle's logic dealt with terms, such as 'all men' and 'mortal', the Stoics developed a logic of whole propositions, connected by words like 'if', 'and' and 'or'. Chrysippus, who reportedly wrote over 700 books, was its great systematizer. The Stoics identified basic valid argument forms called 'indemonstrables', such as: if it is day, it is light; it is day; therefore it is light (now known as modus ponens).

They analyzed conditionals, studied paradoxes like the liar, and distinguished what words mean (lekta, 'sayables') from the sounds themselves. For the Stoics, logic was not just a tool but part of virtue, since a wise person must never assent to a false impression. Because almost all of Chrysippus' works were lost, Stoic logic was undervalued for centuries. In the 20th century, logicians recognized it as an ancient forerunner of propositional logic, which underlies computer circuits.