Page:Popular Science Monthly Volume 86.djvu/84

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IT is well known that sensory stimuli are not always correctly appreciated, and that under certain conditions errors of judgment are made. These errors of perception which are not infrequent with normal people are called illusions and hallucinations. At times, especially in the insane, we find that complex situations and less direct stimuli are apprehended by an individual wrongly, and they lead to the expression of beliefs which are contrary to the experience of others, or opposed to generally accepted beliefs. Some of these false beliefs are called delusions. As usually defined a delusion is considered to be false belief not directly or immediately dependent upon definite sensory stimulation or upon percepts, and in this respect a delusion may be differentiated from an hallucination, a paresthesia and an illusion.

It should be understood, however, that not all false beliefs are delusional in nature. Some are clearly mistakes due to insufficient knowledge. Many years ago it was commonly believed that pelicans fed their young with their own blood. It was also generally held that the sun revolved around the earth. These beliefs were apparently due to lack of knowledge, and although the first scientist who disputed the truth of either of these beliefs opposed the generally accepted belief of the time, his beliefs were later held to be reasonable and not opposed to experience.

Other false beliefs may be due to memory defects. For example, if on a Tuesday a man should say “To-day is Sunday,” the statement would be an expression of a false belief, but the expression of this belief may not in itself be an indication of the presence of a delusion. If the man had only recovered from the prolonged effects of a drug such as alcohol or morphine, under which influence he had been since the preceding Saturday, or if he had just recovered consciousness after a period of unconsciousness of three days, the expression of the belief that the day is Sunday would not in itself indicate that he was deluded. He would have good reason to believe it was Sunday. The most natural and most normal belief he could have under the circumstances would be that he had been drugged or unconscious and that he had just awakened from a period of unconsciousness. The intervening period would be for him the same as if he had been asleep.

In a similar manner mistakes in dates may be made, which are not delusional. Critics tell us that December 25, Christmas Day, is not the date of the birth of the Christ, but a date established in accordance with