8/8 



THE HOMEOSTAT 



(e.g. escape) whose absence would act as ' punishment '. The 

 chief difficulty is that the evidence suggests that the nervous 

 system, especially the mammalian, contains intricate and special- 

 ised mechanisms which give the animals properties not to be 

 deduced from basic principles alone. Thus it has been shown 

 that dogs with an oesophageal fistula, deprived of water for some 

 hours, would, when offered water, drink approximately the 

 quantity that would correct the deprivation, and would then 

 stop drinking; they would stop although no water had entered 

 stomach or system. The properties of these mechanisms have not 

 yet been fully elucidated; so training by reward uses mechanisms 

 of unknown properties. Here we shall ignore these complica- 

 tions. We shall assume that the training is by pain, i.e. by some 

 change which threatens to drive the essential variables outside 

 their normal limits; and we shall assume that training by reward 

 is not essentially dissimilar. 



It should be noticed that in training-experiments the experi- 

 menter often plays a dual role. He first plans the experiment, 

 deciding what rules shall be obeyed during it. Then, when 

 these have been fixed, he takes part in the experiment and obeys 

 these rules. With the first role we are not concerned. In the 

 second, however, it is important to note that the experimenter 

 is now within the functional machinery of the experiment. The 

 truth of this statement can be appreciated more readily if his 

 place is taken by an untrained but obedient assistant who carries 

 out the instructions blindly; or better still if his place is taken by 

 an apparatus which carries out the prescribed actions automatically. 



When the whole training is arranged to occur automatically 

 the feedback is readily demonstrated if we construct the diagram 

 of immediate effects. Thus, a pike in an aquarium was separated 

 from some minnows by a sheet of glass; every time he dashed 

 at the minnows he struck the glass. The following immediate 

 effects can be clearly distinguished: 



