6 INTRODUCTION 



One reason that has led to the adoption of the criterion 

 of associative memory is the evidence afforded that the animal 

 at this stage is guided by experiences of pleasure and pain. 

 We repeat acts that bring us pleasure and avoid those that 

 bring pain. Where in animals certain responses that have 

 been made once tend to be performed more readily on sub- 

 sequent occasions and other acts which are followed by 

 avoiding reactions are discontinued, it is natural to infer 

 that pleasure and pain accompany these different modes 

 of behavior. Pleasure and pain have very commonly been 

 spoken of as agents of reinforcement and inhibition. When 

 the pleasure-pain response appears on the scene conscious- 

 ness is commonly assumed to take a guiding hand in the 

 determination of behavior. The advent of this type of 

 behavior marks a critical period in the evolution of the 

 animal mind and we shall consider it more closely in a sub- 

 sequent chapter. There we shall attempt to show that 

 this type of reaction does not involve the injection of any 

 radically new element into its course of evolution. If we 

 adhere to the doctrine of psycho-physical parallelism in any 

 of its forms we cannot speak, in strictness, of pleasure and 

 pain as agents in accommodation: they are only the signs 

 of a certain kind of adjustment. This adjustment according 

 to the theory of parallelism has its physiological explanation 

 without calling upon the interference of psychical states. 

 We might ask broadly: If psychical states do not as such 

 interfere with the course of physical phenomena, how can 

 we adopt any kind of behavior as a criterion of conscious- 

 ness? I doubt if either Bethe of Loeb is willing to defend 

 the view that consciousness is an agent in directing physical 

 phenomena, and to accept the logical consequences of such 

 a position. It can be shown, I believe, that this view creates 

 many serious difficulties without giving us any real aid in 



