This review was designed to stimulate thinking of the symptoms of mental illness in a physiological frame of reference. If the activity of the cerebral cortex is balanced between mechanisms of reaching out and avoiding, this balance may be disturbed by genetic factors or circumstances developing during life, thus altering the mental set of the individual. Disturbances of cerebral function lead to changes in personality which are related rather specifically to the areas involved. Obsessive, catatonic and oral behavior may be considered as release phenomena. Unilateral injury of the parietal lobe may lead to symptoms of apraxia and agnosia confined to the opposite side of the body. Different types of sensory stimuli are related rather specifically to certain types of motor response.