X.] SENSATION AND THE SENSIFEROUS ORGANS. 255 



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 atten- 

 uated films of their own substance : and that these 

 films, reaching the mind, excite the appropriate sen- 

 sations 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 immaterial, and which they called 

 an " intentional species," effected the needful com- 

 munication 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 defect- 

 ive, 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 

 the physiologist had done their work, is that, between 

 the external object and that mind in which they sup- 

 posed 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 "inten- 

 tional species" cast off by objects, or the "forms "of 



