Certain caveats must be given at the outset. Galen’s most important work in this area, Demonstration, is lost to us except for fragments (though a shorter work, the important Introduction to Logic (Inst. Log.), does survive); and the precise nature of Galen’s innovations, and final views, is a matter of debate amongst experts (for an overview, as well as an account of different views, see Morison 2008). In an area of great technical complexity and interpretive difficulty, we can here attempt no more than the briefest summary of the main issues and points of discussion. These may be said, broadly, to revolve around the questions of the nature and extent of the originality of Galen’s contribution, and of the way in which he draws upon the work of both Aristotle and the Stoics.