How are we to understand Nietzsche’s ubiquitous use of physiological language and imagery in On the Genealogy of Morality? I claim that Nietzsche’s use of physiological language is a crucial element of the method of historical investigation he develops (“genealogy”). If Nietzsche’s genealogy attends to the practices of moral concepts, then the physiological undergoing of those practices will be important data for the genealogist. In other words, in Nietzsche’s critical-historical investigation of morality, accounts of physiological experience will be crucial for having an accurate picture of the practices under investigation. This improved mode of historical investigation sees morality not simply as set of beliefs, but as a lived practice.