The concept of an "idea" played a central role in 17th-century theories of mind and knowledge, but philosophers were divided over the nature of ideas. This book examines an important, but little-known, debate on this question in the work of Leibniz, Malebranche, and Descartes. Looking closely at the issues involved, as well as the particular context in which the debate took place, Jolley demonstrates that the debate has serious implications for a number of major topics in 17th-century philosophy.