Some of the material in Chapters I, II, and V of this volume has already appeared in the Psychological Review; and Chapters XV, XVI, and XVII have been reprinted without very much change. They first appeared as articles in the same journal. In Chapter XXXIII I have made some use of two articles published in the Popular Science Monthly. The chapters on Space and Time are reprinted from the Philosophical Review with little change except that, in Chapter XI, some new matter has been added. To the editors of the journals mentioned, Professor Cattell, Professor Baldwin, and Professor Creighton, my thanks are due for their courtesy in permitting me to reprint as I have done. Thus, about one-fourth of the present volume has already seen the light. It is right that I should say that nothing that has already appeared has been taken up into the book as an afterthought. From the beginning the work has been a unit; it has been for a number of years on my hands, and the publication of the papers above mentioned was due largely to a curiosity to see how the doctrines advocated would impress others. It was, perhaps, hardly fair to present them deprived of their setting, and this injustice, if injustice it be, is remedied now. At the end of the book I have placed a note on the Physical World Order, by my former pupil. Professor Edgar A. Singer, Jr., of the University of Pennsylvania. It has seemed to me of especial interest, as coming from one trained in metaphysical analysis and familiar with the principles and methods of the sciences.