This book articulates and defends the "theory theory" of cognitive and semantic development: the idea that very young children just beginning to talk are engaged in profound restructurings of several domains of their knowledge. These restructurings are analogous to theory changes. The children's early semantic development is closely tied to these theory changes. The relation goes in both directions. The children's cognitive concerns shape and motivate their use of early words. At the same time, attention to the linguistic system itself may reshape their cognition. The first part of the book examines the plausibility of the ideas presented in the book. The second part applies the theory to the specific case of cognitive and semantic development, sketching a tentative account of the successions of theories and theory changes in three important domains--the understanding of appearances, the understanding of actions, and the understanding of kinds--from birth to about age three. The third part of the book examines the implications of the theory for broader questions about language and thought. Finally, the last chapter concentrates on some of the consequences of these views for more general accounts of cognition and meaning in cognitive science and philosophy. Contains over 400 references. (AA)