Cognitive development and various educational implications are discussed in terms of Donald Saari's model of the interaction of a learner and the enviroment and the constraints imposed by the inefficiency of the learner's cognitive system. Saari proposed a hierarchical system of cognitive structures such that the relationships between structures at one level form the structures of the next level. Perceptual identification is the lowest level with simple set formation included in the next level. The third level contains structures corresponding to Piaget's formal operational structures. The learner interprets events on the basis of the structures in his perception of the environment. Each structure has a magnitude that reflects its success in interpreting past events. Interpreting events is assimilation; changing structures is accommodation and learning. Assimilation takes in only a subset of the environment and is not instantaneous. These inefficiencies are discussed in relationship to various aspects of teaching and learning and individual differences. The concept of separation of stimuli is discussed in relation with Witkin's cognitive style dimension of field-dependence-independence and Dunker's functional fixedness. (Author/CTM)