Although there are different opinions about the definition of dreams among researchers who are committed to dreams at home and abroad, it is still difficult to give a satisfactory definition of "dream". We believe that dream is a kind of mental activity caused by the fact that the local brain tissue (cerebral cortex) does not completely stop exciting activities when people sleep. The content of a person's dream is related to the impression of life left in his conscious mind when he is awake, but in dreams, the impression of life often becomes chaotic, so the content of dreams always presents a chaotic and illusory state, which is expressed in the form of imagination or illusion.