been done by me in my _Yoga Philosophy in relation to other Systems of Indian Thought_, the publication of which has for long been unavoidably delayed. All that may be expected from the present volume is that it will convey to the reader the essential features of the Yoga system of thought. How far this expectation will be realized from this book it will be for my readers to judge. It is hoped that the chapter on “Kapila and Pātañjala School of Sāṃkhya” in my _A History of Indian Philosophy_ (Vol. I. Cambridge University Press, 1922) will also prove helpful for the purpose. I am deeply indebted to my friend Mr. Douglas Ainslie for the numerous corrections and suggestions regarding the English style that he was pleased to make throughout the body of the manuscript and the very warm encouragement that he gave me for the publication of this work. In this connection I also beg to offer my best thanks for the valuable suggestions which I received from the reviser of the press. Had it not been for these, the imperfections of the book would have been still greater. The quaintness and inelegance of some of my expressions would, however, be explained if it were borne in mind that here, as well as in my _A History of Indian Philosophy_, I have tried to resist the temptation of making the English happy at the risk of sacrificing the approach to exactness of the philosophical sense; and many ideas of Indian philosophy are such that an exact English rendering of them often becomes hopelessly difficult. I am grateful to my friend and colleague, Mr. D. K. Sen, M.A., for the kind assistance that he rendered in helping me to prepare the index. Last of all, I must express my deep sense of gratefulness to Sir Ashutosh Mookerjee, Kt., C.S.I., etc. etc., and the University of Calcutta, for kindly permitting me to utilize my _A Study of Patañjāli_, which is a Calcutta University publication, for the present work. S. N. DASGUPTA. PRESIDENCY COLLEGE, CALCUTTA, _April, 1924_. CONTENTS BOOK I. YOGA METAPHYSICS: CHAPTER PAGE I. PRAKṚTI 1 II. PURUSHA 13 III. THE REALITY OF THE EXTERNAL WORLD 31 IV. THE PROCESS OF EVOLUTION 40 V. THE EVOLUTION OF THE CATEGORIES 48 VI. EVOLUTION AND CHANGE OF QUALITIES 64 VII. EVOLUTION AND GOD 84 BOOK II. YOGA ETHICS AND PRACTICE: