Jean Piaget's (1896-1980) interdisciplinarity was related to his psychology in several ways. At first, he was simply a tourist of other fields: an interested outsider. But as he became increasingly involved in the professionalization of the discipline, he moved through different contexts that constrained the possibilities for successful action in new and different ways. To make this clear, we adopt a little-known aspect of his later epistemological framework: the open hierarchy of levels. This then provides new perspectives on his life, work, theory, and place in the history of both Swiss and French psychology. It also outlines his reasoning regarding the need for a disciplined approach to interdisciplinary collaboration, institutionalized in the creation of his International Center for Genetic Epistemology. As a result, we not only gain a fuller understanding of how Piaget thought about the development of scientific knowledge, but also how the boundaries of scientific disciplines are being pushed back.