This is not a book upon the Philosophy of Mind, nor does it deal with any philosophical problems. Such problems I have indeed discussed at length in other publications of my own. But the reader of my various philosophical inquiries will already know that I make a sharp difference between the business of the student of philosophy and that of the psychologist. In the present volume, I am concerned solely with certain problems of the natural history of mind; metaphysical issues are here not at all in question. On the other hand, this volume is indeed no effort to summarise the more technical results of modern Experimental Psychology, although I believe thoroughly in the importance of Experimental Psychology, and personally take no small interest in following, so far as I can, the labours of my colleagues of the laboratories; and although I hope that this book shows a good many signs of my having profited by such an interest. My plan has led me, however, to concern myself here with elementary principles rather than with technical details, and to attempt, to some extent, practical applications of these principles, rather than statements of the fascinating, but complex special researches of recent laboratory Psychology.