This volume contains four lectures that Piaget delivered at Columbia University in 1968. In these papers, which summarize Piaget's epistemological research, the classical problem of epistemology is reformulated and simultaneously approached in a manner specific to Piaget: While the philosophical tradition asks about the nature and possibility of knowledge in general, Piaget's guiding perspective is: "How does knowledge come about?" This question implies - in contrast to a speculative philosophical approach - an attempt to explain knowledge through its formation and development.