Also the Upanishads (composed from 700 BC) speak of renewed death (punnarmyrtyu) much before they began speaking of rebirth (punarjanma). However, the problem that the authors of the Upanishads confronted was self-realization, liberation or happiness (moksha). According to the Upanishads, our life is chaos, a dream, while death is order, sleep without dreams because it is final liberation from life, is attaining moksha, which is possible even during the earthly existence, by means of yoga, and consists in the realization of Atman is Brahman.
The clear and explicit mention of the doctrine of transmigration is to be found in the Upanishads. The earliest mention is found in the Brihadaranyaka Upanishad. "A man becomes good by good works, evil by evil' (3. 2.13) and on death, like a caterpillar -or a grass leech - proceeding from one leaf to another, the soul (atman), having shaken off the body and freed itself from ignorance, presumably empirical life, makes a beginning on another body. As a goldsmith forms newer and fairer form from a rough nugget, so the soul fashions for itself another newer, fairer form (4. 4.4), whether it be of the Fathers, or the Gandharvas, or the gods, or Prajapati, or Brahman, or other living beings; just as man acts, just as he behaves, so will he be born. He who does good will be born good, he who does evil will be born evil: he becomes holy by holy deeds, evil by evil (Brahadaranyaka Upanishad 4. 4.2-6; 4. 3 33).
The Chandogya Upanishad 30 speaks of the intimate relation that exits between conduct or action and the condition of rebirth. Human destinies are assigned to two divergent pathways: the pathway of the gods (devas) and the pathway of the fathers or ancestors (pitrs). Those who meditate and practice asceticism follow the pathway of the gods, which leads them (atman) to liberation, to union with Brahman. They are freed forever from the chain of karma-samsara; they will not be reborn. Instead those who walk the normal worldly pursuits follow the pathway of the ancestors, which leads them to rebirth, after having resided in the postmortem realm which lasts as long as the effects of their previous actions have been consumed. If one's good karma predominates over his bad karma, then the soul goes first to hell (place of suffering and purification) for a short period to pay (expiate) his bad karma and then goes to heaven for a longer period, where he enjoys the fruits of his good karma. If, instead, the bad karma predominates, then the soul goes first for a short period to heaven to enjoy the fruits of his good karma and then goes to hell for a longer
period, in order to expiate his bad karma. In both the cases, once the two types of karma are consumed, the soul reincarnates in a place of life determined by the original equilibrium between good karma and bad karma.
According to the Upanishads rebirth or reincarnation of the souls can take place in a series of physical bodies (normally in human bodies but also in animals and even plants) or in a series of astral and preternatural bodies (sun, moon, planets, stars, angels or demons), depending on one s karma. The Kaushitaki Upanishad expresses it more clearly: "He is reborn here either as a worm, or as a butterfly, or as a fish, or as a bird, or as a lion, or as a serpent, or as a tiger, or as a person, or as some other being in this or in that condition, according to his works, according to his knowledge" (1.2)
From the beginning, however, the idea of transmigration was immediately followed by two other ideas: that it was possible for some to be freed of it, and that it was desirable for some to be freed of it. But at the same time, according to some texts of the Upanishads, there was the possibility that some do not want to get out of the chain of transmigration: when the soul of the dead reaches the moon, it can choose to continue in the process of rebirth or be freed of it completely (moksha), and the text affirms that some choose to be reborn (Kaushitaki Upanishad 1.1-7). These two possible options give rise to discussion as to whether it is good or bad to be in the wheel of transmigration.3