The mechanism of mental activity has long puzzled scientists and yielded little more than many tempting, though unproved, theories. This essay presents an unusual point of view. The authors formulate the hypothesis that there is an immaterial structure interposed between the material brain and the mind. This intangible, intermediate brain or "psychic organ" is made of psychoplasm, the psychic equivalent of protoplasm. The ultimate constituent particles, the "psychons," are immeasurably smaller than electrons or protons. The mind of man is, according to this theory, made up of two correlated sets of mechanisms, the material brain with its machinery for receiving sensory impressions, and the psychic brain adapted to synthesize these impressions with psychograms from the memory and present them in consciousness. This hypothesis, with its supporting evidence, may interest physiologists, psychologists and biologists.