X] SENSATION AND THE SENSIFEROUS ORGANS. 251 



immediate instrument by which ideas are presented to the 

 mind j or, in other words, whatever changes are made in this 

 substance, corresponding changes are made in our ideas; and 



Hartley, like Haller, had no conception of the 

 nature and functions of the grey matter of the brain. 

 But, if for " white medullary substance," in the latter 

 paragraph, we substitute "grey cellular substance," 

 Hartley's propositions embody the most probable con- 

 clusions which are to be drawn from the latest 







investigations of physiologists. In order to judge 

 how completely this is the case, it will be well to 

 study some simple case of sensation, and, following the 

 example of Keid and of James Mill, we may begin 

 with the sense of smell. Suppose that I become 

 aware of a musky scent, to which the name of 

 "muskiness" may be given. I call this an odour, 

 and I class it along with the feelings of light, colours, 

 sounds, tastes, and the like, among those phenomena 

 which are known as sensations. To say that I am 

 aware of this phenomenon, or that I have it, or that 

 it exists, are simply different modes of affirming the 

 same facts. If I am asked how I know that it 

 exists, I can only reply that its existence and my 

 knowledge of it are one and the same thing ; in short, 

 that my knowledge is immediate or intuitive, and, as 

 such, is possessed of the highest conceivable degree 

 of certainty. 



1 Ibid. p. 8. The speculations of Bonnet are remarkably similar 

 to those of Hartley ; and they appear to have originated independently, 

 though the "Essai de Psychologie" (1754) is of five years' later date 

 than the "Observations on Man" (1749). 



