86 PSYCHOLOOT. 



Note that in every illusion what is false is what is in- 

 ferred, not what is immediately given. The 'this,' if it 

 were felt by itself alone, would be all right, it only becomes 

 misleading by what it suggests. If it is a sensation of 

 sight, it may suggest a tactile object, for example, which 

 later tactile experiences prove to be not there. The so-called 

 'fallacy of the senses,'' of ivhich the ancient sceptics made so 

 much account, is not fallacy of the senses proper, but rather of 

 the intellect, ivhich interprets ivrongly what the senses give* 



So much premised, let us look a little closer at these 

 illusions. They are due to two main causes. The ivrong 

 object is perceived either because 



1) Although not on this occasion the real cause, it is yet the 

 habitual, inveterate, or most probable cause of ' this ; ' or because 



2) The mind is temporarily full of the thought of that object, 

 and therefore ' this ' is peculiarly prone to suggest it at this 

 moment. 



I will give briefly a number of examples under each 

 head. The first head is the more important, because it 

 includes a number of constant illusions to which all men 

 are subject, and which can only be dispelled by much 

 experience. 



Illusions of the First Type. 



One of the oldest instances dates from Aristotle. Cross 

 two fingers and roll a pea, pen- 

 holder, or other small object be- 

 tween them. It will seem double. 

 Professor Croom Robertson has 

 given the clearest analysis of this 

 illusion. He observes that if 

 Fig. 48. the object be brought into con- 



tact first with the forefinger and next with the second finger, 

 the two contacts seem to come in at difierent points of space. 



* Cf. Th. Reid's Intellectual Powers, essay ii. chap, xxii, and A. Binet, 

 in Mind, ix. 206. M. Binet points out the fact that what is fallaciously 

 inferred is always an object of some other sense than the ' this.' ' Optical 

 illusions ' are generally errors of touch and muscular sensibility, and the 

 fallaciously perceived object and the experiences which correct it are both 

 tactile in these cases. 



