164 evolution: the ages and tomorrow 



of the choice-objects. Also, Weinstein so painted the choice- 

 objects that only a small area was of the appropriate color, 

 the rest being other colors. In this case the subject was ac- 

 tually able to make a choice by not responding to stimuli 

 that lay outside the category, as contrasted to the usual test 

 of this kind where the animal is taught to respond positively 

 to one of the test stimuli. Harlow finds this a much more 

 precise and rigid criterion of concept-formation than any 

 previously apphed to subhuman behavior, being, like human 

 concepts, exclusive as well as inclusive. 



Harlow and other psychologists are convinced that the 

 experimental data now available in the literature clearly 

 show that animals, human and subhuman, must learn to 

 think. The capacity does not appear spontaneously but is 

 the result of a very long and complex learning process— the 

 accumulation of learning sets. The learning set hypothesis 

 gives us a mechanism or principle by which we can explain 

 thinking. 



Psychologists outline the course that learning takes some- 

 what as follows: At the lowest level the individual draws 

 upon unlearned responses or previously learned habits, and 

 as his experience accumulates he discards habits which do 

 not help in the solution of a task and retains and establishes 

 useful habits. At each successive level the individual tries out 

 different types of response to solve a given task, and finally, 

 after having solved many problems of a given kind, he de- 

 velops the organization of responses in patterns designed to 

 meet each particular situation. These are the learning sets, 

 and by extension through the organizing activity of the 

 mind, still more complex combinations of patterns are set up 

 to solve increasingly intricate problems— a process of or- 

 ganizing simple sets into complex units and these into still 

 higher combinations, and so on. 



At the highest level of organization these learning sets be- 

 come innumerable and are interlocked in an incredible com- 

 plexity, a hierarchy that can be manipulated with such ease 



