When we fight an urge, it feels like a strenuous effort, as if there were a homunculus in the head that physically impinged on a persistent antagonist. We speak of exerting will power, of forcing ourselves to go to work, of restraining ourselves and of controlling our temper, as if it were an unruly dog. In recent years the psychologist Roy F. Baumeister has shown that the force metaphor has a kernel of neurobiological reality.