344 AIM AND PROGRESS OF PHYSICAL SCIENCE. 



the sensation of the two eyes could convey to us directly, and 

 without psychical aid, was, at the most, two somewhat different 

 flat pictures of two dimensions as they lay on the two retinae ; 

 instead of this we perceive a representation with three dimen- 

 sions of the things around us. We are sensible as well of 

 the distance of objects not too far removed from us as of their 

 perspective juxtaposition, and compare the actual magnitude of 

 two objects of apparently unequal size at different distances 

 from us with greater certainty than the apparent equal magni- 

 tudes of a finger, say, and the moon. 



One explanation only of our perception of extension in 

 space, which stands the test of each separate fact, can in my 

 judgment be brought forward by our assuming with Lotze that 

 to the sensations of nerve-fibres, differently situated in space, 

 certain differences, local signs, attach themselves, the significa- 

 tions of which, as regards space, we have to learn. That a 

 knowledge of their signification may be attained by these hypo- 

 theses, and with the help of the movements of our body, and 

 that we can at the same time learn which are the right move- 

 ments to bring about a desired result, and become conscious 

 of having arrived at it, has in many ways been established. 



That experience exercised an enormous influence over 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 impulses of this kind is unnecessary, and renders difficult in- 

 stead 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 



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

 the Recent Progress of the Theory of Vision, pp. 175 et seq. 



