  In the Hellenistic period, and apparently independent of Aristotle’s achievements, the logician Diodorus Cronus and his pupil Philo (see the entry  Dialectical school)  worked out the beginnings of a logic that took propositions, rather than terms, as its basic elements. They influenced the second major theorist of logic in antiquity, the Stoic Chrysippus (mid-3rd c.), whose main achievement is the development of a propositional logic, crowned by a deductive system. Regarded by many in antiquity as the greatest logician, he was innovative in a large number of topics that are central to contemporary formal and philosophical logic. The many close similarities between Chrysippus’ philosophical logic and that of Gottlob Frege are especially striking. Chrysippus’ Stoic successors systematized his logic, and made some additions.