Everything in the world has both positive and negative aspects, and human mental activity is no exception; there are normal and abnormal mental activities, resulting in the formation of psychologically normal groups and psychologically abnormal groups. Even for those who are psychologically abnormal, their mental activities are not all abnormal. For example, their personality may be defective in one way or another and be accompanied by thinking disorders, but their feelings and perceptions may be normal. In 400 B.C., Hippocritus, an ancient Greek physician, put forward the idea that the differences in human psychology and personality are closely related to the nature of human bodily fluids to explain the abnormal psychology and behavior of human beings. Between the 5th and 16th centuries, i.e., during the medieval period in Europe, psychological anomalies were viewed exclusively as demonic possession, and so whipping, burning, confinement, and binding were applied across the body of the sufferer as a means of exorcism. If a person is convinced that he sees or hears something, and in the objective world, at that time, there is no stimulus that causes him to feel this way, we can conclude that his mental activity is abnormal and that it produces hallucinations. If the content of a person’s thinking is detached from reality, or the logic of his thinking deviates from the prescriptive nature of objective things and is firmly believed in, we can conclude that his mental activity is abnormal and that he has produced delusions. If a person’s psychological conflicts do not correspond to the actual situation and persist for a long time, unable to extricate themselves, we can conclude that his mental activity is abnormal and he has developed neurotic problems. When a person encounters a pleasant event, he develops pleasant emotions, dances with his hands and joyfully relates his inner experience to others. In this way, we can say that he has normal spirit and behavior. If this is not the case, and in a low tone of voice, he relates pleasant things to others, or reacts happily to painful things, we can say that his mental processes have lost their coherence and consistency, and call it an abnormal state. We also have to suspect an abnormality in the mental activity of a person if there is a problem with the relative stability of his personality in the absence of obvious external causes. This means that we can use the relative stability of personality as one of the criteria for distinguishing between normal and abnormal mental activity. For example, if a person who is very careful with his money suddenly spends it like gold, or if a person who is very warm in his treatment of others suddenly becomes indifferent; if we cannot find in his life circumstances a reason that is sufficient to prompt a change in him, then we can say that his mental activity has deviated from its normal course. In the long course of life, each person develops his own unique personality-psychological traits. Once formed, these psychological traits are relatively stable; in the absence of major external changes, they are generally not easy to change.