1879.] on Sensation. 119 



bility right in terming a vibration) in the epithelium, and this change 

 being transmitted to the nerve fibres, passes along them with a 

 measurable velocity, and, finally reaching the sensorium, is imme- 

 diately followed by the sensation. 



******** 



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 odour, 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' 

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

 immaterial world is a firmer reality than the material. For the sen- 

 sation " 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 contradistinguished from an intuition, and it is a belief 

 which, in any given instance of sensation, may, by possibility, be 

 devoid of foundation. For odours, 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 " subjec- 

 tive " 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 " testi- 

 monies of consciousness," then undoubtedly the testimony of con- 

 sciousness 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 physiology 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 pro- 

 ducing a molecular change in the nerve fibre ; while this last change 

 is also only a mediate cause of sensation, depending, as it does, upon 

 the change which it excites in the sensorium. 



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

 constitute the sensiferous apparatus. They make up the thickness 

 of the wall between the mind, as represented by the sensation " muski- 

 ness," 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 expressible in terms of matter and motion. Whatever changes 



