Indeed, so incontrovertible have these ideas seemed that even the psychologists have long regarded hunger and thirst as biological ·cJrives'. Depending on the theorist, a drive has been defined as the operation of withholding food or water. the resulting physical state of tissue deficit or a general excitation of behaviour that has been induced hy the lack of water or food. A great deal of attention \Vas then paid hy psychologists of learning to the question of how a generic arousal can produce specific behaviour. Not only do \Ve and other mammals suck or bite after food or water deprivation; our ingestive movements can he selectively directed towards the water or calorific materials we need. Physiological psychologists tended to ignore this theoretical problem, however, and assumed that \Vater deficit produced water intake and energy deficit produced food int~1ke without worrying how the behaviour was organized to achieve such outcomes. The psvchologist.s of animal behaviour believed that they had debunked drive theory by showing that rats work hard for the mere taste of saccharin. Physiological psychologists sidestepped that point by imagining that our species survived by getting all the calories needed at critical times from sweet-tasting things. That however is a most implausible principle around which to build the brain (see Chapters 1, 2 and 8).