CHAP. m.J SIGHT. 125 



refer it to its cause in the external world, we can dissociate the 

 two in our minds, and can speak of the mere sensation indepen- 

 dently 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 

 accompany the mere sensations are manifold and become promi- 

 nent 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 knowledge 

 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 psychical 

 visual effect by saying that we ' feel ' a sensation and ' perceive ; 

 an object ; and, though the term ' perception ' 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 ( 702), 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 supposing 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 retinal 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, what they seem ; and one object of philosophy is to ascer- 

 tain 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 which obtain between the actual features of an object 

 and our perception of the object. 



Of these differences some are clearly of psychical origin. Our 

 perception of a tree is in part determined by events other than 

 the actual sensations, by psychical processes arising out of our 

 previous experiences of trees, and in other ways. Some of these 

 psychical processes we shall consider a little later on. 



