300 ON SENSATION AND THE UNITY OF 



back to the Epicurean School and Democritus, 

 and it is to be found fully stated in the fourth 

 book of Lucretius. It comes to this: that the sur- 

 faces of bodies are constantly throwing off exces- 

 sively 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 some- 

 thing, which was not exactly either material or 

 immaterial, 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 de- 

 fective, 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 supposed the 

 sensation to inhere, there lies a physical obstacle. 

 The sense organ is not a mere passage by which 



