244 NATURE AND MAN. 



sents an external object has a real object answering to it. In fact, 

 although we are accustomed to speak of &quot; the evidence of our 

 senses &quot; as worthy of the highest credit, nothing is easier than to 

 show that the evidence of any one sense, without the check 

 afforded by comparison with that of another, is utterly untrust 

 worthy. 



I might pile up instances of visual illusion, for example, in 

 which the subject would be ready to affirm without the slightest 

 hesitation that he sees something which greatly differs from the 

 object that actually forms the picture on his retina ; his erroneous 

 interpretation of that picture being the result of a prepossession 

 derived from antecedent experience. I could show, too, that the 

 same picture may be interpreted in two different modes : a 

 skeleton-diagram, for example, suggesting two dissimilar solid 

 forms, according as the eyes are fixed on one or another of its 

 angles ; and a photograph of a coin or fossil being seen as a 

 cameo or as an intaglio, according as the position of the light 

 affects the interpretation of its lights and shadows. Again, I 

 have before me two pieces of card, A and B, of similar form : 

 when A is placed above B, the latter is unhesitatingly pronounced 

 the larger ; if their relative positions be reversed, A is pronounced 

 with equal conviction, to be the larger ; yet, when one is laid 

 upon the other, they are found to be precisely equal in size. 



So, again, in those more complex combinations of natural 

 objects which the pictorial artist aims to represent, the different 

 modes in which the very same scene shall be treated by two 

 individuals working at the same time and from the same point of 

 view, show how differently they interpret the same visual picture, 

 according to their original constitution and subsequent training. 

 As Carlyle says, &quot; The eye sees what it brings the power to see.&quot; 



But mental prepossessions do much more than this ; they 

 produce sensations having no objective reality. I do not here 

 allude to those &quot; subjective sensations &quot; of physiologists, which 

 depend upon physical affections of nerves in their course, the 

 circulation of poisoned blood in the brain (as in the delirium of 

 fever), and the like ; but I refer to the sensations produced by 

 mental expectancy, a most fertile source of self-deception. The 

 medical practitioner is familiar with these in the case of 



