126 



HABIT AND INTELLIGENCE. 



[chap. 



Account 

 of the 

 process. 



and soiinclp. But experience has taught us that, as a 

 general rule, wherever there is a sensation of sight there 

 is a possible sensation of touch ; and wherever there is a 

 sensation of touch there is a possible sensation of sight. 

 That is to say, if I raise my eyes and see, for instance, my 

 paper-cutter, I know that on stretching forth my hand 

 I am certain to meet with a corresponding sensation of 

 touch ; or if I stretch forth my hand and feel it, I know 

 that on raising my eyes I am certain to meet with a cor- 

 responding sensation of sight. The two sensations are met 

 with together ; and this is our only, but quite sufficient, 

 reason for referring them to the same object.^ 



This truth, that the identification of visual and tangible 

 objects is solely due to experience, was first seen by 

 B erkeley. Berkeley, and is the essential point of liis celebrated theory 

 of vision. He deduced it from purely theoretical data, and 

 it has been subsequently confirmed by observations on 

 persons cured of congenital blindness by the removal of a 

 cataract. It is found that persons under such circum- 

 stances have to learn to see, just as children have to learn 

 to walk ; or, to quote Dr. M'Cosh's expression, they " re- 

 quire observation and thought to reconcile the information 

 they had got from touch, with that which they are now 

 receiving from sight; just as persons who have learned 

 two languages, say German and French, require practice to 

 enable them readily to translate the one into the other." 2 



^ A very remarkable instance of the facility with which what may be 

 called habits of perception are foi-med is afforded by the fact that persons 

 the axes of whose eyes are far from parallel — that is to say, persons who 

 squint — hare the visual images necessarily formed on parts of the two 

 retinas whioli do not correspond, and yet they see single. When the two 

 axes have been set parallel by an operation, so that the two images come 

 to be formed on corresponding parts of the retinas, the patient sees double 

 at first, but soon learns to see single. (Carpenter's Human Physiology, 

 p. 705.) In this case, however, there must be some organic change in the 

 nervous connexion between the two eyes and the optic ganglia. 



2 See the chapter referred to in a former note. It may not be a familiar 

 fact that translation is an art which has to be learned by practice, even 

 when familiarity with two languages exists already ; — in other words, that 

 facility in translating with accuracy from one language into another is not 

 a matter of course, even to those who know both languages well ; — but it 

 is true. 



Quotation 



from 



M'Cosh. 



