42 KNIGHT DUNLAP . 



these cases to refer the image to the mode of the special sense 

 concerned. If the muscles of the eye are involved in the produc- 

 tion of an image, there is a tendency to classify the " image" as 

 visual, and so on. Unquestionably, the use of the vocal organs 

 becomes more pronounced as we advance in age and education, 

 and the "images" which would be naively classed as visual and 

 auditory become less consequential. Olfactory "images" ought 

 at all times to be infrequent, since the muscles involved in the 

 act of smelling are not specialized to that function. 



ii 



The hypothesis presented in the preceding discussion stands 

 upon a postulate which fertile psychological investigation finds 

 increasingly difficult to avoid. This postulate, briefly stated is: 

 habits depend on laws of operation of the nervous system which are 

 the same for habits of perception, habits of thought, and habits of 

 action, because both perception and thought are primarily details 

 in reactions. In this brief formulation importance must be given 

 to the word "primarily," since, as will be explained later, there 

 may be certain features of perception and thought in the final 

 stages of habits of perceiving and thinking, in which they differ 

 from the same habits in the earlier stages. Throughout the 

 field of habit formation we may expect to find that considerable 

 changes hi the mechanism of complicated activities take place 

 in the stages between the primary and the final operations of 

 learning. 



It is postulated, moreover, that consciousness, whether per- 

 ceptual or ideational, depends upon 8 reaction, which is primarily 

 a discharge from receptors, through the central nervous system to 

 effectors, and terminating in specific activity of those effectors. 

 The route over which the discharge travels is designated a reac- 

 tion-arc (the new term being preferable to the older "reflex-arc," 

 which is retained to designate a limited type of reaction-arc). 

 There will be noticed, in the discussion, a certain amount of con- 



8 One may say instead of "depends on," is conditioned by, is caused by, or, is 

 a part of a total process involving : according to one's private brand of philosophy. 



