An Ancient Logic

The present work consists of an attempt to a conceptual reconstruction of the logic of Chrysippus of Soli (cca 279 - cca 206 BCE), from the point of view of modern proof theory. The author argues that Chrysippus and his followers were fully aware of both the truth-value semantics and the proof-theoretical behaviour of the classical logic connectives, and that Chrysippus' logic amounts, actually, to a redundant formulation of classical two-valued logic, understood as a system of rules. From this one can extract several equivalent formulations of classical logic, in natural deduction style, matching modern standards.