CHAP, in.] SIGHT. 931 



occupying a particular position in external nature. Though 

 the latter always accompanies the former, though whenever we 

 experience a visual sensation we refer it to its cause in the 

 external world, we can dissociate the two in our minds, and 

 can speak of the mere sensation independently of the further 

 psychical action. When we have vision not of such a simple 

 object as a luminous point, which we may consider as giving 

 rise to a single sensation, but of a tree which gives rise to a 

 complex group of sensations, the psychical actions which accom- 

 pany the mere sensations are manifold and become prominent 

 in the total visual effect produced by the tree. That total 

 visual effect is determined not only by the sensations to which 

 the retinal image of the tree is at the time giving rise, but also 

 by various psychical events dependent on the previous knowl- 

 edge of the nature of trees which we have gained by touch as 

 well as by sight, and on other circumstances. In common 

 language we distinguish between the mere sensation and the 

 further psj^chical visual effect by saying that we * feel ' a sensa- 

 tion and ' perceive ' an object ; and, though the term ' percep- 

 tion ' has been employed in different meanings by different 

 writers, we may here make use of it, in what is perhaps its most 

 usually accepted meaning, to denote the further visual effect to 

 which we have just called attention as distinguished from the 

 immediate sensation. We feel a sensation of light, and we may 

 feel at one and the same time a number of such sensations of 

 different intensity and quality ; we perceive an object, it may 

 be a simple object such as a mere transient flash of light or a 

 complex object such as a tree or a scene. 



From what we have said above it follows that, although it is 

 perfectly true as we have insisted ( 524 ), that our perception 

 of external objects is based on the optical sharpness of the 

 retinal image, and^on the distinctness of the several sensations 

 which the retinal' image excites, we should be wrong in sup- 

 posing that when an image of an object is formed on the retina 

 the visual impulses correspond exactly to the retinal image, the 

 sensations correspond exactly to the impulses, and the perception 

 corresponds exactly to the sensations, so that the perception is 

 as it were a " mental image " corresponding exactly to the reti- 

 nal image and hence to the object itself. The truth lies in 

 the contrary direction ; things are not what they look, or, since 

 the same applies to other senses besides vision, \\hat they seem ; 

 and one object of philosophy is to ascertain the exact relations 

 between things as they are and things as we think them to be. 

 We must of course confine ourselves here to pointing out, in 

 regard to vision, some of the more salient differences whu-h 

 obtain between the actual features of an object and our percep- 

 tion of the object. 



Of these differences some are clearly of psychical origin. 



