This book is a further contribution to the series Cambridge Studies in Philosophy and Biology. It explains the relationship between intelligence and environmental complexity, and in so doing links philosophy of mind to more general issues about the relations between organisms and environments, and to the general pattern of ÔexternalistÕ explanations. This is a highly original philosophical project that will appeal to a broad swathe of philosophers, especially those working in the philosophy of biology, philosophy of mind, and epistemology.