1879.] on Sensation. 117 



causation. Assuming the existence of an external world, there is no 

 difficulty in obtaining ex2ierimental proof that, as a general rule, 

 olfactory sensations are caused by odorous bodies ; and wo may pass 

 on to the next step of the inquiry — namely, how the odorous body 

 produces the effect attributed to it. 



The first point to be noted hero is another fact revealed by ex- 

 perience ; that the appearance of the sensation is governed, not only by 

 the presence of the odorous substance, but by the condition of a certain 

 part of our corporeal structure, the nose. If the nostrils are closed 

 the presence of the odorous substance does not give rise to the sensa- 

 tion ; while, when they are oj^en, the sensation is intensified by the ap- 

 proximation of the odorous substance to them, and by snuffing up the 

 adjacent air in such a manner as to draw it into the nose. Ou the 

 other hand, looking at an odorous substance, or rubbing it on the 

 skin, or holding it to the ear, does not awaken the sensation. Thus, 

 it can be readily established by experiment that the perviousness of 

 the nasal passages is, in some way, essential to the sensory function ; 

 in fact, that the organ of that function is lodged somewhere in the 

 nasal passages. And, since odorous bodies give rise to their effiicts at 

 considerable distances, the suggestion is obvious that something must 

 pass from them into the sense organ. What is this something which 

 plays the. part of an intermediary between the odorous body and the 

 sensory organ ? 



The oldest speculation about the matter dates back to Democritus 

 and the Epicurean School, and it is to be found fully stated in the 

 fourth book of Lucretius. It comes to this: that the surfaces of 

 bodies are constantly throwing off excessively attenuated films of 

 their own substance ; and that these films, reaching the mind, excite 

 the appropriate sensations in it. 



Aristotle did not admit the existence of any such material films, 

 but conceived that it was the form of the substance, and not its 

 matter, which affected sense, as a seal impresses wax, without 

 losing anything in the process. While many, if not the majority, 

 of the Schoolmen took up an intermediate position, and supposed 

 that a something which was not exactly either material or im- 

 material, and which they called an " intentional species,'* effected the 

 needful communication between the bodily cause of sensation and the 

 mind. 



But all these notions, whatever may be said for, or against, them 

 in general, are fundamentally defective, by reason of an oversight 

 which was inevitable, in the state of knowledge at the time in which 

 they were promulgated. What the older philosophers did not know, 

 and could not know, before the anatomist and physiologist had done 

 his work, is that, between the external object and that mind in 

 which they supposed the sensation to inhere, there lies a physical 

 obstacle. The sense organ is not a mere passage by which the 

 " tenuia simulacra rerum," or the " intentional species " cast off by 



