(a) The person has only extrinsic needs

      One broad question in psychology is 'What is the function of the nervous system?' and one broad answer provided by the theory of evolution is 'to enable the organism to survive?' The next question is 'How does the nervous system enable the organism to survive?' and the classic answer is 'It ensures that the organism will approach things that are good for it (e.g. things that it eats) and avoid things that are bad for it (e.g. things that eat it).' The need-reduction theory describes the former mechanism and the activation theory describes the latter mechanism. The need-reduction theory and the activation theory are the means employed by behaviorists to fit psychology within the theory of evolution. Let us look briefly at each theory in turn.

      You are alive. You are in a precarious state. Life is a narrow tightrope with death on either side. To stay alive, you must maintain yourself within a narrow range of temperature, blood-sugar concentration, metabolic rate, and so on. The process by which you maintain yourself in this optimal state, in which those indices are neither too low nor too high, is called homeostasis. Let us imagine you have just been wined and dined. You are in your optimal state. However, you cannot remain thus for long. The mere passage of time conspires against your bliss. You get thirsty. You get hungry. This physiological state of deprivation is called a need. The need can be satisfied by appropriate behavior with respect to some appropriate thing in the environment (the goal) - drinking liquid in the case of thirst and eating food in the case of hunger. Since the nervous system is the only system which 'knows' the environment, the physiological need in the digestive system must be transformed into some psychological counterpart in the nervous system. This psychological counterpart (the drive) orients you to the appropriate goal.~ By making the appropriate response with respect to the goal, the drive is removed, the need is satisfied, and the optimal state is regained.