For thousands of years extraordinary and supernatural phenomena have been studied, investigated, and generalised, the whole ground of the religious faculties of man has been analysed, and the practical result is the science of Raja Yoga. In the modern times Swami Vivekananda has been the most authoritative oracle to propound this age-old science of God-realization. The main theme of the book is concentration of mind as a means to superconscious state. The first part of the book comprises several lectures to classes delivered by the Swami in New York. The second part is a rather free translation of the aphorisms of Patanjali, with a running commentary. This is a book invaluable for all the practitioners of meditation.