This condition is termed "psychic blindness;" and what is there lost is not the power of vision, but of interpreting, of assimilating, of reading the meaning of visual sense-impressions. It is this interpreting and assimilating process that is largely concerned in the formation of illusions. In the experiences of daily life we have to do not with simple 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 continually and persistently overlooked. It is an occasional experience in raising a water-pitcher to have the vessel fly up in the hand in a very startling manner,--the reason for this being that the pitcher, which one is accustomed to find well filled, happens to be empty. This experience shows that we unconsciously estimate the force necessary to raise the vessel, but only become conscious of this train of inference when it happens to lead to conclusions contradictory of the fact. The perception of distance, once thought to be as primitive a factor in cognition as the impression of a color, is likewise the result of complex though unrealized inferences; and the phenomena of the stereoscope, by imitating the conditions of the perception of solidity and thus making us see as solid the flat representations of a pair of diagrams or photographs, furnish a brilliant illustration of the variety and complexity of such unconscious reasonings. As for essential purposes normal persons have a common anatomy, a common physiology, and a common psychology, it results that we draw these unconscious inferences after the same pattern; and so completely are they the outcome of the normal reactions to our common environment, that we need not be, and as a rule are not, aware of their existence until--and probably with some little effort--our attention is directed to them. Unconsciously and spontaneously we learn to see,--that is, to extract meaning out of the sense-impressions that fall upon the retina.