122 HABIT AND INTELLIGENCE. [cHAP. 



have no doubt the three actually co-operate in producing 

 it. These are — 



1. The existence of two or more sensations in different 

 places at the same time. 



2. A sensation changing its place. 



3. Our own motions. 



Sight, like AVe coguise space, or extension, by the sense of sight 

 miSiiill '^^ ^^^ ^^ ^y ^^^^ ^^ touch ; and in sight, as in touch, we 

 cognises are originally cognizant of superficial extension only, 

 superficial What we Originally see is only coloured surface ; and 

 extension, -what we are originally cognizant of in seeing is only 

 Percep- sensations of colour, and the mutual relations of those 

 bi°htl^ an seusations in superficial space. The power of perceiving — 

 acquh-ed that is to sav, of inferring — the existence of external 



power. 1 • , 1 1-1 ■ T 



objects by sight, as by touch, is a later acquired power. 

 Facts con- This view, that the cognition of space is at first of 

 these superficial space, is, of course, not capable of proof. But 



views. jt ig very strongly supported by a fact of which every 

 one's consciousness will inform him ; namely, that it is 

 easier to think of extension as superficial than either 

 as linear or as solid. If we desire to think of a line, 

 we imagine a surface and draw the line on it. If we 

 desire to think of space of three dimensions, we imagine 

 a surface, and then, I think with some little difi&culty, 

 add the thuxl dimension. Such a fact as this may be com- 

 pared to those facts of organization which have no bearing 

 on the functions of the adult, but are records of the 

 process of its development. It may be said that on my 

 theory of the acquisition of the cognition of space by the 

 co-existence of similar sensations in different parts of the 

 sentient surface of the body, there is no reason why our 

 first thoughts of extension should be superficial rather 

 Answer to than Knear. I reply, that in practice we become cognizant 

 o yec on. ^^ extension, not by the co-existence of two separated sen- 

 sations, but by the co-existence of an indefinite number 

 of sensations, some of them similar and some unlike, 

 spread over the surface of the body ; and thus our first 

 perceptions of extension are not linear, bu.t superficial. 

 I have as yet spoken only of perceptions by a single 



