Many of the teachings of the ancient Stoics were nearly identical to those of modern propositional logic. For example, Philo of Megara (fl. 300 BC) said that a conditional is false if it has a true antecedent and a false consequent; in all other cases it is true. Diodorus (died 284 BC) taught that a conditional is true if and only if it neither is nor was possible for the antecedent to be true and the consequent false.
Is that cool or what?