BEGINNINGS OF INTELLIGENCE 171 
movements. The latter problem is one whose solution 
appears hopeless. If we accept the doctrine of psycho- 
physical parallelism in any of its forms, we must deny that 
psychic states are, strictly speaking, the causes of physical 
changes. Why then should pleasure be connected with one 
kind of activity and pain with another? Why not just the 
reverse? This problem is, I believe, insoluble, because it is 
a question of the relation of the physical and the psychical; 
it is of essentially the same nature as the question why one 
kind of retinal stimulation produces a sensation of red and 
another a sensation of green. Physical and psychical states 
are correlated in particular ways; this we accept as a matter 
of observed connection. But why a certain kind of brain 
vibration is associated with a state of consciousness we call 
a sensation of red instead of some other state is a question 
upon which we may intend our minds indefinitely without 
the least profit. If we adopt any other theory of the relation 
of mind and body we are in no way better off. If we have 
to do with a preordained connection of pleasure with certain 
physiological activities and pain with certain others, this 
connection is no more intelligible if we admit the interaction 
of psychical and physical states than it is under the theory 
of parallelism. We can only say that such is the observed 
relation of the phenomena. 
What is feasible to attempt to solve’is the problem of the 
adaptive modification of behavior which we may say is the 
objective side of the pleasure-pain process. We are dealing 
with a series of physiological reactions and how they come 
to be modified. We may assume that psychical states enter 
into the chain of causes and effects that make up an animal’s 
behavior, but it is not clear that such an assumption throws 
the least light upon our problem, and it is open to serious 
objections on both scientific and metaphysical grounds. We 
