72 PSYCHOLOGY. 



degree of sound fainter still tlian tlie preceding pianissimo 

 This phenomenon is not confined to hearing : 



" If we slowly approach our finger to a surface of water, we often 

 deceive ourselves about the moment in which the wetting occurs. The 

 apprehensive patient believes himself to feel the knife of the surgeon 

 whilst it is still at some distance." * 



Visual perception supplies numberless instances in which 

 the same sensation of vision is perceived as one object or 

 another according to the interpretation of the mind. Many 

 of these instances will come before us in the course of the 

 next two chapters ; and in Chapter XIX similar illusions 

 will be described in the other senses. Taken together, all 

 these facts would force us to admit that the subjective 

 difference hetiveen imagined and felt objects is less absolute 

 than has been claimed, and that the cortical processes which 

 underlie imagination and sensation are not quite as discrete 

 as one at first is tempted to suppose. That peripheral sen- 

 sory processes are ordinarily involved in imagination seems 

 improbable ; that they may sometimes be aroused from the cortex 

 doionwards cannot, hoivever, be dogmatically denied. 



The imagination-process can then pass over into the sensa- 

 tion-process. In other words, genuine sensations can be 

 centrally originated. When we come to study hallucina- 

 tions in the chapter on Outer Perception, we shall see that 

 this is by no means a thing of rare occurrence. At present, 

 however, we must admit that normally the two processes do 

 NOT pass over into each other ; and we must inquire why. 

 One of two things must be the reason. Either 



1. Sensation-processes occupy a different locality from 

 imagination-processes ; or 



2. Occupying the same locality, they have an intensity 

 which under normal circumstances currents from other 

 cortical regions are incapable of arousing, and to produce 

 which currents from the periphery are required. 



It seems almost certain (after what was said in Chapter 

 II. pp. 49-51) that the imagination-process differs from the 

 sensation-process by its intensity rather than by its locality. 

 However it may be with lower animals, the assumption that 



* Lotze, Med. Psych, p. 509. 



