In this article, Kieran Egan contests the scientific foundations of Piaget's developmental theories and the scientific basis of much educational research. In so doing, he pushes researchers and practitioners alike to rethink the centrality of Piaget's tenets to teaching and learning. Egan traces the history of the developmental literature that preceded Piaget. In particular, he examines the thoughts of Rousseau and Spencer regarding cognitive development, and how the ideas of both men informed those of Piaget. Throughout, Egan critiques the notion that these developmental theories are based on empirical evidence. Through this critique, he enters the current debate on the role of scientific inquiry in educational research and practice.