In a previous paper it has been pointed out that these physiological drives or needs are to be considered unusual rather than typical because they are isolable, and because they are localizable physically. That is to say, they are relatively independent of each other, of other motivations and of the organism as a whole, and secondly, in many cases, it is possible to demonstrate a localized, underlying physical base for the drive. This is true less generally than has been thought (exceptions are fatigue, sleepiness, maternal responses) but it is still true in the classic instances of hunger, sex, and thirst.
