Stoic Propositional Logic: A New Reconstruction

Abstract

<p>I reconstruct Stoic propositional logic, from the ancient testimonies, in a way somewhat different than the 10 reconstructions published before 2002, building especially on the work of Michael Frede (1974) and Suzanne Bobzien (1996, 1999). In the course of reconstructing the system, I draw attention to several of its features that are rarely remarked about, such as its punctuation-free notation, the status of the premisses of an argument as something intermediate between a set and a sequence of propositions, the incorrectness of the almost universal translation of the Greek label for the primitives of the system as indemonstrable arguments, the probable existence of an extended set of primitives which accommodates conjunctions with more than two conjuncts and disjunctions with more than two disjuncts, the basis for the system’s exclusion of redundant premisses, and the reason why the hypothetical syllogisms of Theophrastus are not derivable in the system. I argue that, though sound according to its originator’s (Chrysippus’s) conception of validity, the system as reconstructed is not complete according to that conception. It is an open problem what one needs to add to the system in order to make it Chrysippean-complete, or even whether it is possible to do so without making it Chrysippean-unsound.</p>