This book provides a new theory of the nature of nonlinguistic thought. Many scientific disciplines treat nonlinguistic creatures as thinkers, explaining their behavior in terms of their thoughts about themselves and about the environment. But this theorizing has proceeded without any clear account of the types of thinking available to nonlinguistic creatures. One consequence of this is that ascriptions of thoughts to nonlinguistic creatures have frequently been held to be metaphorical and not to be taken at face value. The book offers a conceptual framework for treating human infants and nonhuman animals as genuine thinkers. Whereas existing discussions of thought at the nonlinguistic level have concentrated on how such thoughts might be physically realized, it approaches the problem by considering what is required in explaining behavior in psychological terms. In developing a positive account of nonlinguistic thought the book shows how the experimental tools used by developmental psychologists and students of animal behavior can be used to give a precise account of the way in which a human infant or nonhuman animal is representing the world. Much of the book is devoted to exploring the differences between thinking without words and language-based thinking. The book argues that there are clear limits to the expressive power of nonlinguistic thought. Nonetheless, it identifies primitive analogues at the nonlinguistic level that can be used to explain sophisticated nonlinguistic behaviors.