Had it not been for the labours of the later commentators, much of what is found in Patañjali’s aphorisms would have remained extremely obscure and doubtful, at least to all those who were not associated with such ascetics as practised them, and who derived the theoretical and practical knowledge of the subject from their preceptors in an upward succession of generations leading up to the age of Patañjali, or even before him. It is well to bear in mind that Yoga is even now practised in India, and the continuity of traditional instruction handed down from teacher to pupil is not yet completely broken. If anyone wishes methodically to pursue a course which may lead him ultimately to the goal aimed at by Yoga, he must devote his entire life to it under the strict practical guidance of an advanced teacher. The present work can in no sense be considered as a practical guide for such purposes. But it is also erroneous to think—as many uninformed people do—that the only interest of Yoga lies in its practical side. The philosophical, psychological, cosmological, ethical, and religious doctrines, as well as its doctrines regarding matter and change, are extremely interesting in themselves, and have a definitely assured place in the history of the progress of human thought; and, for a right understanding of the essential features of the higher thoughts of India, as well as of the practical side of Yoga, their knowledge is indispensable. The Yoga doctrines taught by Patañjali are regarded as the highest of all Yogas (_Rājayoga_), as distinguished from other types of Yoga practices, such as _Haṭhayoga_ or _Mantrayoga_. Of these _Haṭhayoga_ consists largely of a system of bodily exercises for warding off diseases, and making the body fit for calmly bearing all sorts of physical privations and physical strains. _Mantrayoga_ is a course of meditation on certain mystical syllables which leads to the audition of certain mystical sounds. This book does not deal with any of these mystical practices nor does it lay any stress on the performance of any of those miracles described by Patañjali. The scope of this work is limited to a brief exposition of the intellectual foundation—or the theoretical side—of the Yoga practices, consisting of the philosophical, psychological, cosmological, ethical, religious, and other doctrines which underlie these practices. The affinity of the system of Sāṃkhya thought, generally ascribed to a mythical sage, Kapila, to that of Yoga of Patañjali is so great on most important points of theoretical interest that they may both be regarded as two different modifications of one common system of ideas. I have, therefore, often taken the liberty of explaining Yoga ideas by a reference to kindred ideas in Sāṃkhya. But the doctrines of Yoga could very well have been compared or contrasted with great profit with the doctrines of other systems of Indian thought. This has purposely been omitted here as it has already