As an apostle of Indian thought in the West, however, the Swami's labours were of a somewhat more complex character. Here we find him, in the numerous works which he has left, not fully defining and expanding the great basic philosophy of Adwaita or Unity the idea of the Immanent Divine,but also, as in the case of the present volume, acting as a witness to the authentic of an antique form of knowledge, which, familiar as it is to India can scarcely be regarded as known to Europe even by name.
Apart from its obvious division into an original treatise and the translation of an Oriental work and its commentators, this book of Raja Yoga falls under a twofold category. In the first place, we find ourselves listening as it were to a melody which identifies the subject with religion, and in the second to an intermingled strain by which it is regarded purely as science. On one side, we hear the impassioned cry, "The way is found! Children of immortality, and ye who dwell in higher spheres, by perceiving Him who is beyond all darkness, your path is made from out of this darkness. And to escape, ye have no other!" And on the other hand, as we follow page after page, and comment upon comment, we feel that at least as regards temper, apart from the question of credibility, we are in the presence of nothing more or less than an ancient and unfamiliar system of psychology, complete of its own kind, and supported by a vocabulary and system of reasoning curiously unlike any to which we are accustomed.
......Book review by Sister Nivedita - Swami Vivekananda's Introduction to Raja Yoga