146 THE POPULAR SCIENCE MONTHLY. 



unless lie become blind in early cbildbood, he will retain a good 

 memory for visual images, will be able to more or less clearly 

 imagine pictorially the appearances of objects from verbal descrip- 

 tions, and in the free roamings of his dream fancy will live in a 

 world in which blindness is unknown. On the other hand, cases 

 occur where, owing to the disordering of certain portions of the 

 finely organized cortex of the brain, the patient, though retaining 

 full sight and understanding, is unable to derive any meaning from 

 what he sees. The same group of sensations that suggest to our 

 minds a book, a picture, a face, and all the numerous associations 

 clustering about these, are as unmeaning to him as the symbols of 

 a cipher alijhabet. This condition is termed " psychic blindness," 

 and what is there lost is not the power of vision, but of interpret- 

 ing, of assimilating, of reading the meaning of visual sense-im- 

 pressions. 



In the experiences of daily life we seldom have to do with sim- 

 ple sensations, but with more or less complex inferences from them ; 

 and it is just because these inferences go on so constantly and so 

 unconsciously that they are so continually and so persistently 

 overlooked. It has probably happened to the reader that, upon 

 raising a pitcher of water which he was accustomed to find well 

 filled, the vessel has flown up in his hand in a very startling man- 

 ner. The source of the difficulty was the emptiness of the pitcher. 

 This shows that one unconsciously estimates the force necessary 

 to raise the vessel but only becomes conscious of this train of 

 inference when it happens to lead to conclusions contradictory of 

 the fact. The perception of distance, long thought to be as primi- 

 tive a factor in cognition as the impression of a color, is likewise 

 the result of complex inferences ; and the phenomena of the stereo- 

 scope furnish unending illustrations of the variety and complexity 

 of these unconscious reasonings. These, it must be noted, are 

 drawn by all persons alike ; but, like the man who was unaware 

 that he had been talking prose all his life until so informed, are 

 not recognized as such until special attention is directed to them. 



The simplest type of a deception occurs when such an inference 

 owing to an unusual disposition of external circumstances, leads 

 to a conclusion which other and presumably superior testimony 

 shows to be false. A typical case is the observation, described 

 already by Aristotle, that a ball or other round object held between 

 two fingers crossed one over the other, will seem double. Under 

 ordinary circumstances a sensation of contact on the left side of 

 one finger and on the right side of the finger next to it (to the 

 right) could only be produced by the simultaneous application of 

 two bodies. We unconsciously make the same inference when the 

 fingers are crossed and thus fall into error — an error, it is impor- 

 tant to observe, which we do not outgrow but antagonize by 



