142 MENTAL EVOLUTION IN ANIMALS. 



CHAPTER X. 



Imagination. 



We have already considered the psychology of Ideation to 

 the extent of defining the sense in which I employ the word 

 " Idea" or " Image," and also to the extent of tracing, both on 

 the side of physiology and on that of psychology, the prin- 

 ciple of the association of ideas.* We have now to analyze 

 the psychology of Ideation somewhat more in detail. 



The simplest case of an idea is the memory of a sensa- 

 tion. That a sensation may be remembered even when there 

 has been no perception is proved, not only by the fact before 

 mentioned that an infant only a day or two old can distin- 

 guish a change of milk, but also by the fact, which must be 

 familiar to all, that several minutes after an unperceived 

 sensation is past, we are able by reflection to remember that 

 we have had the sensation. For example, a man reading a 

 book may hear a clock strike from one to five strokes (or 

 perhaps more) without perceiving the sound, yet a minute or 

 two afterwards he can recall the past sensation and tell the 

 number of strokes which have occurred. And in simpler 

 instances the memory of a sensation may extend over a much 

 longer time. 



The simplest case of an idea, then, being the memory of a 

 past sensation (as distinguished from the memory of a past 

 perception), it follows that the earliest stages of ideation 

 must be held to correspond with those earlier stages of 

 memory which we have already described, wherein as yet 

 there is no association of ideas, but merely a perception of 

 a present sensation as like or unlike a past one. Hence 

 in its most elementary form an idea may be said to consist 

 in the faint revival of a sensation. This view has already 

 been advanced with much clearness by Mr. Spencer, Professor 



* Soc Chapters II and III. 



