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 the 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 
