The second chapter of Aitareya Upanishad gives a detailed description of the three births of man. It relates about the need of liberating one`s self from all desires to obtain the true knowledge.
The individual self at first becomes an embryo. He remains within the seeds and bears his own self. After he is shed into the female he receives his first birth. After entering into the self of the female he is being nourished continuously until his birth. The mother bears him in her womb and takes care of him. In return he too nourishes his own self to continue the worlds. It is in this manner that the world is sustained. This is thus regarded as his second birth.
This self of him i.e. the son, being the self of the father is made to take his place for sacred works. Then after having handed over his duties to the son the other self accomplished all his duties and thus leaves this world at a high age. Thus having left it, he is born again. This is his third birth. The Rishi thus says that, `Within the womb, I know well all the births of those gods; hundreds of bodies, strong as iron, kept me; looking downwards, like a falcon, I shall be gone in an instant.` Thus, upon knowing that after the destruction of this body, being elevated from this world and having obtained all desires in the place of heaven, he becomes immortal.
Thus, from the above explanation it can be said that the first birth is the condition of man in the shape of a seed, of a foetus; the second birth his condition when he is born; and the third his state, when departing from this world to be born again in a new body. The consequence of ignorance about the worldly state of the soul while proceeding from one birth to another liberates him from transmigration and thus becomes immortal. Man is empowered with knowledge and becomes one with the Supreme Self, obtaining all desires in the heavenly world and become immortal through the rebirth of the Atman.
