Vico’s constructivist epistemology is compared with that of Piaget with a view to clarifying Piaget’s theory of knowledge. Piaget’s interpreters often show a lack of concern with the metaphysical foundations of cognitive structures. Vico’s emphasis on the limitations of human knowledge, therefore, is helpful in avoiding interpretive inconsistency. In Vico’s and in Piaget’s radical constructivism, knowledge is non-ontological in the sense that no claims may be made about the relation between cognitive structures and reality. Structural adequacy is derived from the consistency of the self-referencing cognitive system.