322 MENTAL EVOLUTION IN ANIMALS. 



cases, to an exceptional case" — i.e., the illusion arises from an 

 erroneous inference. It therefore seems needless for me to 

 occupy space with an enumeration of instances. 



The first or earliest stage of inference, then, is that in 

 (k. which the inference arises in or together with the perception, 

 as when we infer that a gnat is, a bird, or that the portion of 

 a surface corresponding to the blind spot of the retina is 

 coloured like the surrounding portions of the surfa ce ; infe- 

 rence may liere be said to be a constituent part of perception.* 

 In other words, we do not in such cases really sensate all that 

 we perceive, and the residue of the perception is supplied by 

 inference which is unconscious only because it is so instan- 

 taneous. The reason why in such cases it is so instantaneous, 

 is because the part furnished by inference has been so 

 habitually associated with the part furnished by sensation, 

 that the instant the sensation is perceived the mental addition 

 is supplied. That this is the true explanation of the matter 

 is rendered evident, not only from the deductive considera- 

 tions just stated, but also from the inductive verification 

 which they receive from the facts that arise when a man who 

 has been born blind is suddenly made to see. A good case of 

 this kind is the celebrated one of the youth (about twelve years 

 of age) whom Mr. Cheselden couched for removing congenital 

 cataracts from both eyes. I shaU therefore quote a few pas- 

 sages from Mr. Cheselden's account of the case. 



" When he first saw he was so far from making any judg- 

 ment about distances, that he thought all objects whatever 

 touched his eyes (as he expressed) as what he felt touched 

 his skin, and thought no objects so agreeable as those which 

 were smooth and regular, though he could form no judgment 

 of their shape, or guess what it was in any object that was 

 pleasing to him. He knew not the shape of anything, nor 

 any one thing from another, however different in shape and 

 magnitude ; but upon being told what things were, whose 

 form he before knew from feeling, he would carefully observe, 

 that he might know them again; but having too many 

 objects to learn at once, he forgot many of them ; and (as he 

 said) at first learnt to know, and again forgot a thousand 

 things in a day. One particular only (though it may appear 

 trifling) I will relate. Having often forgotten which was the 



* Just in the same way as we found perception to form an integral part 

 of Memory and of tlie Association of Ideas. 



