With the doctrine that true knowledge led to supreme bliss by the absorption of the individual soul in Brahma went hand in hand the theory of transmigration (samsara). That theory is developed in the oldest Upanishads; it must have been firmly established by the time Buddhism arose, for Buddha accepted it without question. Its earliest form is found in the Çatapatha Brahmana, where the notion of being born again after death and dying repeatedly is coupled with that of retribution. Thus it is here said that those who have correct knowledge and perform a certain sacrifice are born again after death for immortality, while those who have not such knowledge and do not perform this sacrifice are reborn again and again, becoming the prey of Death. The notion here expressed does not go beyond repeated births and deaths in the next world. It is transformed to the doctrine of transmigration in the Upanishads by supposing rebirth to take place in this world. In the Brihadaranyaka we further meet with the beginnings of the doctrine of karma, or "action," which regulates the new birth, and makes it depend on a man's own deeds. When the body returns to the elements, nothing of the individuality is here said to remain but the karma, according to which a man becomes good or bad. This is, perhaps, the germ of the Buddhistic doctrine, which, though denying the existence of soul altogether, allows karma to continue after death and to determine the next birth. The most important and detailed account of the theory of transmigration which we possess from Vedic times is supplied by the Chhandogya Upanishad. The forest ascetic possessed of knowledge and faith, it is here said, after death enters the devayana, the "path of the gods," which leads to absorption in Brahma, while the householder who has performed sacrifice and good works goes by the pitriyana or "path of the Fathers" to the moon, where he remains till the consequences of his actions are exhausted. He then returns to earth, being first born again as a plant and afterwards as a man of one of the three highest castes. Here we have a double retribution, first in the next world, then by transmigration in this. The former is a survival of the old Vedic belief about the future life. The wicked are born again as outcasts (chandalas), dogs or swine. The account of the Brihadaranyaka (VI. ii. 15-16) is similar. Those who have true knowledge and faith pass through the world of the gods and the sun to the world of Brahma, whence there is no return. Those who practise sacrifice and good works pass through the world of the Fathers to the moon, whence they return to earth, being born again as men. Others become birds, beasts, and reptiles.