AIM AKD PROGEESS OF PHYSICAL SCIENCE. 393 



the signification of visual pictures, and, in cases of doubt, 

 is generally the final arbiter, is allowed even by those 

 physiologists who wish to save as much as possible of the 

 innate harmony of the senses with the external world. 

 The controversy is at present almost entirely confined to 

 the question of the proportion at birth of the innate 

 impulses that can facilitate training in the understanding 

 of sensations. The assumption of the existence of im- 

 pulses of this kind is unnecessary, and renders difficult 

 instead of elucidating an interpretation of well-observed 

 phenomena in adults. 1 



It follows, then, that this subtile and most admirable 

 harmony existing between our sensations and the objects 

 causing them is substantially, and with but few doubtful 

 exceptions, a conformity individually acquired, a result 

 of experience, of training, the recollection of former acts 

 of a similar kind. 



This completes the circle of our observations, and lands 

 us at the spot whence we set out. We found at the 

 beginning, that what physical science strives after is 

 the knowledge of laws, in other words, the knowledge how 

 at different times under the same conditions the same 

 results are brought about ; and we found in the last 

 instance how all laws can be reduced to laws of motion. 

 We now find, in conclusion, that our sensations are merely 

 signs of changes taking place in the external world, and 

 can only be regarded as pictures in that they represent 

 succession in time. For this very reason they are in a 

 position to show directly the conformity to law, in regard 

 to succession in time, of natural phenomena. If, under 

 the same natural circumstances, the same action take 

 place, a person observing it under the same conditions 

 will find the same series of impressions regularly recur. 



1 A further exposition of these conditions will be found in the lectures on 

 the Recent Progress of the Theory of Vision, pp. 197 et seq. 



