We have given the physiological name of the "phronema," as the real organ of mind or the instrument of reason, to that part of the cortex on the normal anatomic condition of which the action of the human mind depends. The remarkable investigations during the last few decades of the finer texture of the grey cortex (or cortical substance of the cerebrum) have shown that its structure--a real anatomic "wonder of life"--represents the most perfect morphological product of plasm; and its physiological function--mind--is the most perfect action of a "dynamo-machine," the highest achievement that we know anywhere in nature. Millions of psychic cells or neurona--each of them of an extremely elaborate fibril molecular structure--are associated as special thought-organs (phroneta) at certain parts of the cortex, and these again are built up into a large harmonious system of wonderful regularity and capacity. Each phronetal cell is a small chemical laboratory, contributing its share to the unified central function of the mind, the conscious action of reason. Scientists are still very far from agreement as to the extent of the phronema in the cortex and its delimitation from the neighboring sense-centres (sensoria). But they are all agreed that there is such a central organ of mind, and that its normal anatomic and chemical condition is the first requisite for the life of the human mind. This belief--one of the foundations of monistic psychology--is confirmed by the study of psychiatry. The study of the diseased organism has greatly furthered our knowledge of the normal frame. Diseases are so many physiological experiments made by nature herself under special conditions, which experimental physiology would often be unable to arrange artificially. The thoughtful physician or pathologist can often obtain most important knowledge of the function of organs by carefully observing them during disease. This is especially true of diseases of the mind, which always have their immediate foundation in an anatomical or chemical modification of certain parts of the brain. Our advancing knowledge of the localization of mental functions, or of their connection with special phroneta or organs of thought, is for the most part based on the experience that the destruction of the one is followed by the extinction of the other. Modern psychiatry, the empirical science of mental disease, has thus become an important element of our monistic psychology. If Immanuel Kant had studied it and had visited the asylum wards for a few months, he would certainly have escaped the dualist errors of his philosophy. We may say the same of the modern metaphysical psychologists who built up a mystic theory of an immortal soul without knowing the anatomy, physiology, and pathology of the brain.