AbstractThis paper focuses on the influence of the language of medical diagnosis, prognosis and treatment at an important juncture in the inquiry into psuchē Plato undertakes in the Republic: the discussion of degenerate regimes and forms of soul in Books 8 and 9. I argue that the work that is accomplished by Plato’s critical appropriation of medical terms and concepts is crucial to the dialogue as a whole: it gives us insight into the deepest questions and problems animating the dialogue, namely, how to choose the best life possible, and what role philosophy has in this task. More specifically, I argue that the medically inflected psychological and political diagnostics that Socrates employs in the analysis of civic and psychic decay in Books 8 and 9 produces a sophisticated critical theory, one which takes as its object of analysis neither the city nor the soul by themselves, but rather the peculiar degenerating operation of desire that is common to both, and which hinges upon the capacity for self-analysis of the diagnostician. This critical theory is further developed by the fusion of medical and juridical stances that is operative in the judgment of lives with which Book 9 concludes.