Arguments for an extension of Piaget’s theory of cognitive development have been derived from philosophical and historical consideration of modern natural sciences. Implicit contradictions, which characterize these sciences as well as common thought, can be systematically apprehended only through a dialectic reinterpretation. The dialectic basis of Piaget’s theory is expressed in his assimilation-accommodation paradigm. But development is interpreted as a continuing alienation from this basis culminating in the noncontradictory thinking of formal operations. Although Piaget’s interpretations capture a rich variety of performances during childhood, they fail to represent adequately the thought and emotions of mature and creative persons. For an interpretation of adulthood and aging, a return to the dialectic basis is necessary. Such a reorganization can proceed from any of the four major levels of development. It introduces intra- and interindividual variations into Piaget’s theory. Individuals may operate simultaneously or in short succession at different cognitive levels. The ceaseless striving toward formal operations becomes inappropriate and ineffective for the level of dialectic maturity.