This book deals with a previously neglected episode in the history of logic and theories of cognition: the way in which conceptions of inference changed during the 17th century. Gaukroger focuses on the work of Descartes, contrasting his explanation of inference as an instantaneous grasp in accord with the natural light of reason with the Aristotelian view of inference as a discursive process. He offers a new interpretation of Descartes' contribution to the question, revealing it to be a significant advance over humanist and late Scholastic conceptions, and argues that the Cartesian account played a pivotal role in the development of our understanding of the nature of inference.