Of all the philosophical doctrines advocated by Galen in his vast oeuvre, the Platonic theory of the tripartite soul is among those to which he is most strongly committed. The practice of medicine demands a theory of the soul, especially one that emphasizes its role in accounting for physiological functions. This chapter assesses Galen's argument for the tripartite theory as presented in De Placitis Hippocratis et Platonis (PHP). It discusses the basis of Galen's division of the soul and describes the psychological and physiological aspects of his theory. The distinction between parts and powers is crucial for understanding the basic argumentative structure of PHP. Galen's argument rests on a theory of scientific demonstration (apodeixis) that he describes and deploys in a wide range of works. In Galen's view, the human intellect and sense organs when functioning normally serve as "natural criteria" that yield facts or propositions that are self-justifying.