258 PSYCHOLOGY. 



replaced by an imagined solid ivTiich we describe as seen. 

 Really it is not seen hut only so vividly conceived as to 

 approach a vision of reality. We feel all the while, however, 

 that the solid suggested is not solidly there. The reason 

 why one solid may seem more easily suggested than 

 another, and ivhy it is easier in general to perceive the 

 diagram solid than fiat, seems due to probability.* Those 

 lines have countless times in our past experience been 

 drawn on our retina by solids for once that we have seen 

 them flat on paper. And hundreds of times we have 

 looked down upon the upper surface of parallelopipeds, 

 stairs and glasses, for once that we have looked upwards 

 at their bottom — hence we see the solids easiest as if from 

 above. 



Habit or probability seems also to govern the illusion of 

 the intaglio profile, and of the hollow mask. We have never 

 seen a human face except in relief — hence the case with 

 which the present sensation is overpowered. Hence, too, 

 the obstinacy with which human faces and forms, and 

 other extremely familiar convex objects, refuse to appear 

 hollow when viewed through Wheatstone's pseudoscope. 

 Our perception seems wedded to certain total ways of 

 seeing certain objects. The moment the object is suggested 

 at all, it takes possession of the mind in the fulness of its 

 stereotyped habitual form. This explains the suddenness 

 of the transformations when the perceptions change. The 

 object shoots back and forth completely from this to that 

 familiar thing, and doubtful, indeterminate, and composite 

 things are excluded, apparently because we are unused to 

 their existence. 



When we turn from the diagrams to the actual folded 

 visiting-card and to the real glass, the imagined form seems 

 fully as real as the correct one. The card flaps over ; the 

 glass rim tilts this way or that, as if some inward spring 

 suddenly became released in our eye. In these changes the 

 actual retinal image receives difl'erent complements from the 

 mind. But the remarkable thing is that the complement 



* Cf. E. Mach, Beitrage, etc., p. 90, and the preceding chapter of the 

 present work, p. 86 ff. 



