In the conclusion to the first book of the Treatise, Hume's skeptical reflections have plunged him into melancholy. He then proceeds through a complex series of stages, resulting in renewed interest in philosophy. Interpreters have struggled to explain the connection between the stages. I argue that Hume's repeated invocation of the four humors of ancient and medieval medicine explains the succession, and sheds a new light on the significance of skepticism. The humoral context not only reveals that Hume conceives of (...) skepticism primarily as a temperament, not a philosophical view or system. It also resolves a puzzle about how Hume can view skepticism as both an illness and a cure. The skeptical temperament can, depending on its degree of predominance, either contribute to or upset the balance of temperaments required for proper mental functioning. (shrink)