Psychiatry made a great advance when it began to recognize the psychological meaning of mental symptoms—when it took the stand that it is not enough to establish that a patient has, say, delusions, but that one must relate the content of the delusions to the patient's life experience. Another stride has yet to be made, and that is the recognition of the physiological meaning of symptoms. Mental activity being the manifestation of cerebral activity, mental aberration must signify some aberration in the function of the brain, however normal this organ may appear to the eye. In a case of mental disorder, therefore, just as much as in hemiplegia, one must inquire how the laws of physiology reveal themselves in the signs and symptoms of the disease. To return to the example, it is not enough to show that the content of a delusion represents the patient's thoughts and strivings, but there remains the question: What has happened to his brain to cause his thoughts and strivings to assume the guise of delusions, when in a normal man they merely take the form of fancies? When a deluded patient says he is a very rich man, a certain psychological cause is at work. A healthy man, too, may be worried about money, but, in response to this cause, he merely fancies himself a rich man. The demonstration of a psychological cause, therefore, does not explain the sick man's delusion. All it explains is the content of the delusion; since the patient is worried about money, his delusion deals with wealth rather than some other topic. But it does not explain why the patient has delusions. The explanation of this must lie in some cerebral defect which permits inferior modes of thought to occur in response to certain situations. Psychiatry will not reach its full stature as a science until it regards each mentally sick person and each of his symptoms as a problem in cerebral physiology.