This volume grew out of work on Berkeley which was presented in a dissertation several years ago. Though now much revised and greatly expanded. particularly in respect of the theory of concepts, a good part of the present text rests on this earlier foundation. I therefore gladly take this opportunity to express my appreciation to my teachers both at Indiana University and at McGill, and especially to Professor Newton Stallknecht who directed my dissertation. For permission to quote from the Berkeley manuscripts in their keeping, I have first to thank the Trustees of the British Museum, and the Board of Trinity College Dublin. I wish further to thank the Bodleian Library, Oxford for allowing me to quote from their collection of Locke manu- scripts. Also I am grateful to the Editor of Filoso/ia for letting me use excerpts from an article that first appeared in the Stu'di Internazionali di Filoso/ia, and to George Allen and Unwin. Publishers, for permission to quote a long passage from Bertrand Russell's Analysis 0/ Mind. From thesis project to published book, my research on the Berkeley manuscripts has been made possible by the generous and timely support of the Canada Council. Finally. I wish to thank Mrs. Anne Hillier for preparing the manuscript with great patience and skill.