386 STUDIES IN SPECIAL SENSE PHYSIOLOGY" 



may be traced through Hering to Goethe, and the latter almost 

 explicitly founded his work on that of Aristotle. 



It is true that the study of visual psychology received a consider- 

 able impetus at the beginning of the eighteenth century from the 

 work of Berkeley. But Berkeley's writings are perhaps rather a 

 contribution to epistemology than to physiological .psychology as 

 we now understand it. 1 



I shall therefore pass to the development of our knowledge as 

 to the actual process of stimulation, so far as experiment has 

 tended to display it, and the theoretical developments this has 

 received in modern times. We can then again take up the thread 

 of the narrative treating of visual sensations and attempt an 

 estimate of its modern outcome. Before leaving this pre- 

 liminary sketch, however, it is well to point out one difficulty 

 in framing an hypothesis of vision, which hampered the acutest 

 thinker of the Greek period, but from which we have pa'rtially 

 escaped. 



It will have been seen that practically all these philosophers 

 either explicitly or implicitly adopted the postulate that like acts 

 upon like. Anaxagoras might appear to be an exception, but a 

 little thought convinces one that the exception is only formal. 

 In adopting the literal converse of the proposition he too is com- 

 mitted to the belief that there is some necessary connection in 

 kind between the processes occurring within the eye or the mind 

 and those supposed to exist outside of it. This idea pervades all 

 the Greek speculations thus, since the external medium is trans- 

 parent, there must be some internal transparency ; since " fire " 

 is visible, there must be some internal " fire " by which it is 

 perceived, and so forth. In fact this " similarity hypothesis " 

 might be regarded as the most primitive of all forms of specula- 

 tion. There is reason to believe that sympathetic or homoeopathic 

 magic, an obvious extension of the same idea, is culturally older 

 than any religions, while most of the latter embody the conception 

 in some more or less changed form. 



The real importance of what is called Miiller's Law of Specific 

 Sense Energy, is that it contains an explicit denial of any necessary 



1 For Berkeley's views on colour, see "The First Dialogue between Hylas and 

 Philonous" (vol. i., p. 314 et seq.). " Alciphoron, " Fourth Dialogue (Vol. ii. 

 p 288, &c.). Cf. also "An Essay towards a New Theory of Vision" (vol. i:, p. 79 

 et seq.). The references are to Sampson's edit.'on (Bell, 1898). 



