Three senses of ‘dialectic’ can be discerned in Galen's works (section 10.1): (1) Dialectic as logic, i.e., analysis and techniques of arguments. (2) Dialectic as a part of logic that focuses on certain kinds of arguments characterized by the status of their premisses, which are different from scientific ones. (3) Dialectic as the analysis of names or words and their meanings in ordinary language. Galen’s account of logic and dialectic presents similarities and differences with Alcinous' and Alexander, but is distinctively connected to his views about the epistemic status of medicine. Section 10.2 focuses on Galen’s discussion of Aristotle’s eternalist cosmology, in book 4 of his lost treatise On Demonstration. It is similar to that of the Platonist Atticus but highly original: it can be seen as a discussion pro et contra the eternity of the cosmos, showing the fallacies of speculative knowledge. The third section focuses on Galen’s account of common notions and definitions. It is compared with that of Stoic and Platonist philosophers. Once again, Galen’s account appears to be distinctively connected to his views about the characteristics and scope of scientific knowledge.