24 ORIGIN OF PHYSICAL CONCEPTS 



emitted Sounds by which they were recognised ; 

 it is not doubtful that these would then come to 

 be employed by us as the guides of our Activity 

 and would acquire in our minds the character 

 of Extensity. They would arrange themselves in 

 a cotemporaneous, extensive, or spatial relation to 

 one another just as the objects of Vision do at 

 present. 



It is only, therefore, when we come to employ 

 the simultaneous presentation of Vision as the 

 instrument of our Activity and the guide of Action 

 that it acquires the character commonly called 

 extensive. Successive visual sensations convey 

 no extensive suggestion. 



It is important to realise the nature of this 

 peculiar feature in the data of Vision. The sounds 

 which we hear, the odours which we smell, are the 

 immediate result of certain undulations affecting 

 the appropriate organ of sensation. We refer these 

 to the object in which the undulations originate. 

 In like manner a light which we see is referred to 

 its objective luminous source. But light also and 

 in addition is reflected from, and thus reveals the 

 presence of the whole body of our resistant environ- 

 ment. Hence is derived the coloured presenta- 

 tion of Vision to which the character of extensity 

 attaches. Nothing similar takes place in the case 



