unless he become blind in early childhood, 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 descriptions, 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 alphabet. 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.
In the experiences of daily life we seldom have to do 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 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 manner. 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 primitive a factor in cognition as the impression of a color, is likewise the result of complex inferences; and the phenomena of the stereoscope 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 important to observe, which we do not outgrow but antagonize by