SENSATION AND SENSIFEROUS ORGANS. 89 



of the same kind with the oscillations of pendulums and the tremblings of the 

 particles of sounding bodies. They must be conceived to be exceedingly short 

 and small, so as not to have the least efficacy to disturb or move the whole bodies 

 of the nerves or brain.* 



The white medullary substance of the brain is also the immediate instrument 

 by which ideas are presented to the mind ; or, in other words, whatever changes 

 are made in this substance, corresponding changes are made in our ideas ; and 

 vice versa.i 



Hartley, like Haller, had no conception of the nature and functions 

 of the gray matter of the brain. But, if for " white medullary sub- 

 stance," in the latter paragraph, we substitute "gray cellular sub- 

 stance," Hartley's propositions embody the most probable conclusions 

 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 Reid 

 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 " muski- 

 ness " may be given. I call this an odor, and I class it along with the 

 feelings of light, colors, sounds, tastes, and the like, among those phe- 

 nomena 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 differ- 

 ent 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. 



The pure sensation of muskiness is almost sure to be followed by a 

 mental state which is not a sensation, but a belief, that there is some- 

 where close at hand a something on which the existence of the sensa- 

 tion depends. It may be a musk-deer, or a musk-rat, or a musk-plant, 

 or a grain of dry musk, or simply a scented handkerchief; but former 

 experience leads us to believe that the sensation is due to the presence 

 of one or other of these objects, and that it will vanish if the object is 

 removed. In other words, there arises a belief in an external cause 

 of the muskiness, which, in common language, is termed an odorous 

 body. 



But the manner in which this belief is usually put into words is 

 strangely misleading. If we are dealing with a musk-plant, for ex- 

 ample, we do not confine ourselves to a simple statement of that which 

 we believe, and say that the musk-plant is the cause of the sensation 

 called muskiness ; but we say that the plant has a musky smell, and 

 we speak of the odor as a quality, or property, inherent in the plant. 



* " Observations on Man," vol. i., p. 11. 



f Ibid., p. 8. The speculations of Bonnet are remarkably similar to those of Hart- 

 ley ; and they appear to have originated independently, though the " Essai de Psycho- 

 logie" (1754) is of five years' later date than the " Observations on Man " (1749). 



