What is that by knowing which everything in this vast universe is known? This question has sometime or other agitated all thinking persons. The hoary sages of ancient India, after deep and prolonged meditation, discovered the answer that by knowing Atman, the sole reality that sustains the universe, all is known; for the atman creates this universe and enters into it as soul. Atman also termed Brahman, the creator is the supreme soul; atman the created is the individual soul. The entire esoteric fabric of the Upanishads, which number more than a hundred, is woven around two concepts-that of the Brahman and the atman. They urge the earnest seeker to strive for Brahma-atma-aikyam (unity of the Brahman and the Atman). As the path to this knowledge is beset with perils and sharp as a razor's edge, the adept teachers commencing from such an insignificant trifle as a fig take their pupils through theology, cosmology, psychology and eschatology to that destination where all illusions vanish and the purport of laconic but profound statements like tat tvam asi (that art thou), aham brahma asmi (I am Brahman) are realized. Countless philosophers, notable being Badarayana the author of Brahma - sutras, have sought inspiration from the upanishads for their systems of philosophy. The unabating popularity of Paul Deussen's The Philosophy of the Upanishads ever since its first publication in 1906 attests to the quality of its contents. This second edition is brought out to reach a wider circle of readers who desire to have a close acquaintance with the philosophy of the Upanishads.