In attempting to characterize successive stages in the development of thinking, one has often made the distinction between concrete, situational thinking and abstract, conceptual or categorial thinking. In the first the person's thinking and acting are directly determined by the immediate situation in its concreteness and singularity; in the second the person tries to master the situation by applying to it certain categories or concepts. These two ways of perceiving the world, which have repeatedly been described as the two major stages in the development both of the individual person and of society or culture, have been used also for characterization of certain pathologic conditions. Gelb and Goldstein have convincingly demonstrated that in patients with cerebral lesions there is a striking loss of the conceptual approach to the world, or of the "categorial attitude," as they termed it. They showed how this loss accounts both for changes in the patient's