CHAPTER VIII 
PLEASURE, PAIN, AND THE BEGINNINGS OF 
INTELLIGENCE’ 
“Apprehensio sensitiva non attingit ad communem rationem 
boni, sed ad aliquod bonum particulare, quod est delectabile. Et 
ideo secundum appetitum sensitivum, qui est in animalibus, opera- 
tiones quaeruntur propter delectionem.””—Tuomas Aquinas, Summa 
Theologica. 
Psychologists nowadays with comparatively few exceptions 
agree in regarding intelligence not as a faculty standing in 
sharp contrast to instinct, as was formerly taught, but as one 
resting on a foundation of instinct, and gradually growing 
out of behavior of the purely instinctive type.. The term 
intelligence is used here in the wider sense as embracing all 
those forms of profiting by experience through the formation 
of associations. It therefore includes psychic activity rang- 
ing from simple associative memory to complex trains of 
reasoning. What distinguishes intelligence from instinct is 
that in the latter the connections between acts are based 
upon hereditary organization, whereas in the former they 
are. established through experience. The apparently new 
thing involved in intelligent behavior is the power of form- 
ing associations. So far as we can judge of the psychic states 
of an animal from its behavior, animal intelligence in its first 
manifestations consists in repeating acts which bring pleasure 
and in avoiding things which cause pain, and a discussion of 
the transition from instinct to intelligence naturally involves 
'This chapter is taken with some modifications from an article of the 
same title contributed by the writer to the Journal of Comparative Neu- 
rology and Psychology. I am indebted to the editor for permission to 
reprint a considerable part of the article here. 
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