The really distinctive part of the system is the establishment of the views prevailing in Patanjali's time with regard to asceticism and the mysterious powers to be acquired by its practice. Yoga, or "yoking" the mind, means mental concentration on a particular object. The belief that fasting and other penances produce supernatural powers goes back to remote prehistoric times, and still prevails among savage races. Bodily asceticism of this kind is known to the Vedas under the name of tapas. From this, with the advance of intellectual life in India, was developed the practice of mental asceticism called yoga, which must have been known and practised several centuries before Patanjali's time. For recent investigations have shown that Buddhism started not only from the theoretical Sankhya but from the practical Yoga doctrine; and the condition of ecstatic abstraction was from the beginning held in high esteem among the Buddhists. Patanjali only elaborated the doctrine, describing at length the means of attaining concentration and carrying it to the highest pitch. In his system the methodical practice of Yoga acquired a special importance; for, in addition to conferring supernatural powers, it here becomes the chief means of salvation. His Sutras consist of four chapters dealing with deep meditation (samadhi), the means for obtaining it (sadhana), the miraculous powers (vibhuti) it confers, and the isolation (kaivalya) of the redeemed soul. The oldest and best commentary on this work is that of Vyasa, dating from the seventh century A.D. Many of the later Upanishads are largely concerned with the Yoga doctrine. The lawbook of Manu in Book VI. refers to various details of Yoga practice. Indeed, it seems likely, owing to the theistic point of view of that work, that its Sankhya notions were derived from the Yoga system. The Mahabharata treats of Yoga in considerable detail, especially in Book XII. It is particularly prominent in the Bhagavadgita, which is even designated a yoga-çastra. Belief in the efficacy of Yoga still prevails in India, and its practice survives. But its adherents, the Yogis, are at the present day often nothing more than conjurers and jugglers. The exercises of mental concentration are in the later commentaries distinguished by the name of raja-yoga or "chief Yoga." The external expedients are called kriya-yoga, or "practical Yoga." The more intense form of the latter, in later works called hatha-yoga, or "forcible Yoga," and dealing for the most part with suppression of the breath, is very often contrasted with raja-yoga.