The first full-length study of Jean Piaget as a philosopher and evolutionist. Messerly traces Piaget's earliest conjectures about knowledge through its further developments to its mature formulation as 'genetic epistemology.' Messerly analyzes Piaget's constructivist theory of the evolution of human knowledge as continuous with, yet partially transcending, the biological process of adaptation to the environment. Messerly's study serves as an invitation to further explorations with Paiget's theory and will interest philosophers, biologists, and psychologists.