A broad theory of depression is proposed based on a blend of Hume's (D. Hume, 1978, A Treatise of Human Nature. Oxford, England: Oxford University Press. Original work published 1739) concept of a valenced world and Damasio's (A. R. Damasio, 1994, Descartes' Error: Emotion, Reason, and the Human Brain. New York, NY: Putnam) somatic marker hypothesis. It is proposed that behavior is governed by a hedonic detector system coupled to working memory. Depression is assumed to reflect a malfunction of the hedonic detector, involving the inappropriate setting of its positive−negative neutral point. This simple assumption allows a coherent account of depression that links the neurobiological, behavioral, and psychological evidence with a plausible evolutionary hypothesis. Alternative hypothesis are discussed, and methods of investigating the hedonic detector are suggested.