Yoga is the steadiness of the mind, with the annihilation of the mental states, into a particular type of graduated state leading to Self-realization. The earliest exponent of this system called Rajayoga, the highest of all yogas as distinguished from Hathayoga and Mantrayoga was Patanjali who wrote Aphorisms on Yoga in 150 B.C. The present treatise relates to the system of Rajayoga as enunciated by Patanjali and explained by his commentators Vyasa, Vacaspati, Vijnanabhiksu and others. It deals with the foundation of the yoga practices their philosophical, psychological, cosmological, ethical and religious doctrines. It compares the essential features of the yoga system with other systems of philosophical thought and establishes, in particular, its affinity to the system of Samkhya. The work consists of fifteen chapters divided into two parts bound in one volume. Part I (Chapters I-VII) deals with Yoga Metaphysics, delineating mainly the characteristics and functions of Prakrti and Purusa, the reality of the external world and the process of Evolution. Part II (Chapters VIII-XV) expounds Yoga Ethics and Practice with emphasis on Yoga method stages of Samadhi and kindred topics. This study is documented with a preface giving an outline of the topic, an appendix on Sphota-the relation of words with their ideas and things, and a general index. Contents Book I. Yoga Metaphysics: Prakrti, Purusha, The Reality of the External World, The Process of Evolution, The Evolution of the Categories, Evolution and Change of Qualities, Evolution and God. Book II. Yoga Ethics and Practice: Mind and Moral States, The Theory of Karma, The Ethical Problem, Yoga Practice, The yogangas, Stages of Samadhi, God in Yoga, Matter and Mind, Appendix, Index.