This thesis is a comparative study in the History of Modern Philosophy focused on the recourse to philosophical physiology on the part of two key figures, Spinoza and Nietzsche. This involves comparative research into their emphatic appeal to the body as the key to solving fundamental philosophical problems. While the groundwork for comparative research has already been laid in studies of a number of key concepts, a comparative study of Spinoza and Nietzsche focused on physiology has not yet been conducted. I argue that, in spite of tremendous differences, these two thinkers come close in their rejection of moral and metaphysical illusions they claim are characteristic of the philosophical tradition they respond to, and in their focus on self-knowledge understood as knowledge of the body’s endogenous power to act. This leads them to formulate strikingly similar normative programs informed by their shared c ommitment to naturalism and immanence.