SENSATION AND SENSIFEROUS ORGANS. 93 



then it is quite possible that the particular mode of motion of the object 

 is reproduced in the sensorium ; exactly as the diaphragm of a telephone 

 reproduces the mode of motion taken up at its receiving- end. In other 

 words, the secondary " intentional species " may be, as the schoolmen 

 thought the primary one was, the last link between matter and mind. 



None the less, however, does it remain true that no similarity exists, 

 nor indeed is conceivable, between the cause of the sensation and the 

 sensation. Attend as closely to the sensations of muskiness, or any 

 other odor, as we will, no trace of extension, resistance, or motion is 

 discernible in them. They have no attribute in common with those 

 which we ascribe to matter ; they are, in the strictest sense of the 

 words, immaterial entities. 



Thus, the most elementary study of sensation justifies Descartes's 

 position, that we know more of mind than we do of body; that the im- 

 material world is a firmer reality than the material. For the sensation 

 "muskiness" is known immediately. So long as it persists, it is a part 

 of what we call our thinking selves, and its existence lies beyond the 

 possibility of doubt. The knowledge of an objective or material cause 

 of the sensation, on the other hand, is mediate ; it is a belief as contra- 

 distinguished from an intuition ; and it is a belief which, in any given 

 instance of sensation, may, by possibility, be devoid of foundation. 

 For odors, like other sensations, may arise from the occurrence of the 

 appropriate molecular changes in the nerve or in the sensorium, by the 

 operation of a cause distinct from the affection of the sense-organ by an 

 odorous body. Such " subjective " sensations are as real existences as 

 any others, and as distinctly suggest an external odorous object as their 

 cause ; but the belief thus generated is a delusion. And, if beliefs are 

 properly termed " testimonies of consciousness," then undoubtedly the 

 testimony of consciousness may be, and often is, untrustworthy. 



Another very important consideration arises out of the facts as they 

 are now known. That which, in the absence of a knowledge of the physi- 

 ology of sensation, we call the cause of the smell, and term the odorous 

 object, is only such, mediately, by reason of its emitting particles which 

 give rise to a mode of motion in the sense-organ. The sense-organ, 

 again, is only a mediate cause by reason of its producing a molecular 

 change in the nerve-fiber; while this last change is also only a mediate 

 cause of sensation, depending, as it does, upon the change which it ex- 

 cites in the sensorium. 



The sense-organ, the nerve, and the sensorium, taken together, con- 

 stitute the sensiferous apparatus. They make up the thickness of the 

 wall between the mind, as represented by the sensation "muskiness," 

 and the object, as represented by the particle of musk in contact with 

 the olfactory epithelium. 



It will be observed that the sensiferous wall and the external world 

 are of the same nature ; whatever it is that constitutes them both is ex- 

 pressible in terms of matter and motion. Whatever changes take place 



