When the Vedic people witnessed the burning of a dead body they supposed that the eye of the man went to the sun, his breath to the wind, his speech to the fire, his limbs to the different parts of the universe. They also believed as we have already seen in the recompense of good and bad actions in worlds other than our own, and though we hear of such things as the passage of the human soul into trees, etc., the tendency towards transmigration had but little developed at the time.
In the Upani@sads however we find a clear development in the direction of transmigration in two distinct stages. In the one the Vedic idea of a recompense in the other world is combined withthe doctrine of transmigration, whereas in the other the doctrine of transmigration comes to the forefront in supersession of the idea of a recompense in the other world. Thus it is said that those who performed charitable deeds or such public works as the digging of wells, etc., follow after death the way of the fathers (_pit@ryana_), in which the soul after death enters first into smoke, then into night, the dark half of the month, etc., and at last reaches the moon; after a residence there as long as the remnant of his good deeds remains he descends again through ether, wind, smoke, mist, cloud, rain, herbage, food and seed, and through the assimilation of food by man he enters the womb of the mother and is born again. Here we see that the soul had not only a recompense in the world of the moon, but was re-born again in this world [Footnote ref 84].
The other way is the way of gods (_devayana_), meant for those who cultivate faith and asceticism (_tapas_). These souls at death enter successively into flame, day, bright half of the month, bright half of the year, sun, moon, lightning, and then finally into Brahman never to return. Deussen says that "the meaning of the whole is that the soul on the way of the gods reaches regions of ever-increasing light, in which is concentrated all that is bright and radiant as stations on the way to Brahman the 'light of lights'" (_jyoti@sa@m jyoti@h_) [Footnote ref 85].
The other line of thought is a direct reference to the doctrine of transmigration unmixed with the idea of reaping the fruits of his deeds (_karma_) by passing through the other worlds and without reference to the doctrine of the ways of the fathers and gods, the _Yanas_. Thus Yajnavalkya says, "when the soul becomes weak (apparent weakness owing to the weakness of the body with which it is associated) and falls into a swoon as it were, these senses go towards it. It (Soul) takes these light particles within itself and centres itself only in the heart. Thus when the person in the eye turns back, then the soul cannot know colour; (the senses) become one (with him); (people about him) say he does not see; (the senses) become one (with him), he does not smell, (the senses) become one (with him), he does not taste, (the senses) become one (with him), he does not speak, (the senses) become one (with him), he does not hear, (the senses) become one (with him), he does not think, (the senses) become one with him, he does not touch, (the senses) become one with him, he does not know, they say. Thetip of his heart shines and by that shining this soul goes out. When he goes out either through the eye, the head, or by any other part of the body, the vital function (_pra@na_) follows and all the senses follow the vital function (_pra@na_) in coming out. He is then with determinate consciousness and as such he comes out. Knowledge, the deeds as well as previous experience (_prajna_) accompany him. Just as a caterpillar going to the end of a blade of grass, by undertaking a separate movement collects itself, so this self after destroying this body, removing ignorance, by a separate movement collects itself. Just as a goldsmith taking a small bit of gold, gives to it a newer and fairer form, so the soul after destroying this body and removing ignorance fashions a newer and fairer form as of the Pit@rs, the Gandharvas, the gods, of Prajapati or Brahma or of any other being....As he acts and behaves so he becomes, good by good deeds, bad by bad deeds, virtuous by virtuous deeds and vicious by vice. The man is full of desires. As he desires so he wills, as he wills so he works, as the work is done so it happens. There is also a verse, being attached to that he wants to gain by karma that to which he was attached. Having reaped the full fruit (lit. gone to the end) of the karma that he does here, he returns back to this world for doing karma [Footnote ref 86]. So it is the case with those who have desires. He who has no desires, who had no desires, who has freed himself from all desires, is satisfied in his desires and in himself, his senses do not go out. He being Brahma attains Brahmahood. Thus the verse says, when all the desires that are in his heart are got rid of, the mortal becomes immortal and attains Brahma here" (B@rh. IV. iv. 1-7).
A close consideration of the above passage shows that the self itself destroyed the body and built up a newer and fairer frame by its own activity when it reached the end of the present life. At the time of death, the self collected within itself all senses and faculties and after death all its previous knowledge, work and experience accompanied him. The falling off of the body at the time of death is only for the building of a newer body either in this world or in the other worlds. The self which thus takes rebirth is regarded as an aggregation of diverse categories. Thus it is said that "he is of the essence of understanding,of the vital function, of the visual sense, of the auditory sense, of the essence of the five elements (which would make up the physical body in accordance with its needs) or the essence of desires, of the essence of restraint of desires, of the essence of anger, of the essence of turning off from all anger, of the essence of dharma, of the essence of adharma, of the essence of all that is this (manifest) and that is that (unmanifest or latent)" (B@rh. IV. iv. 5). The self that undergoes rebirth is thus a unity not only of moral and psychological tendencies, but also of all the elements which compose the physical world. The whole process of his changes follows from this nature of his; for whatever he desires, he wills and whatever he wills he acts, and in accordance with his acts the fruit happens. The whole logic of the genesis of karma and its fruits is held up within him, for he is a unity of the moral and psychological tendencies on the one hand and elements of the physical world on the other.
The self that undergoes rebirth being a combination of diverse psychological and moral tendencies and the physical elements holds within itself the principle of all its transformations. The root of all this is the desire of the self and the consequent fruition of it through will and act. When the self continues to desire and act, it reaps the fruit and comes again to this world for performing acts. This world is generally regarded as the field for performing karma, whereas other worlds are regarded as places where the fruits of karma are reaped by those born as celestial beings. But there is no emphasis in the Upani@sads on this point. The Pit@ryana theory is not indeed given up, but it seems only to form a part in the larger scheme of rebirth in other worlds and sometimes in this world too. All the course of these rebirths is effected by the self itself by its own desires, and if it ceases to desire, it suffers no rebirth and becomes immortal. The most distinctive feature of this doctrine is this, that it refers to desires as the cause of rebirth and not karma. Karma only comes as the connecting link between desires and rebirth--for it is said that whatever a man desires he wills, and whatever he wills he acts.
Thus it is said in another place "he who knowingly desires is born by his desires in those places (accordingly), but for him whose desires have been fulfilled and who has realized himself, all his desires vanish here" (Mu@n@d III. 2. 2). This destruction of desires is effected by the right knowledge of the self. "He who knows his self as 'I am the person' for what wish and for what desire will he trouble the body,...even being here if we know it, well if we do not, what a great destruction" (B@rh. IV. iv. 12 and 14). "In former times the wise men did not desire sons, thinking what shall we do with sons since this our self is the universe" (B@rh. IV. iv. 22). None of the complexities of the karma doctrine which we find later on in more recent developments of Hindu thought can be found in the Upani@sads. The whole scheme is worked out on the principle of desire (_kama_) and karma only serves as the link between it and the actual effects desired and willed by the person.
It is interesting to note in this connection that consistently with the idea that desires (_kama_) led to rebirth, we find that in some Upani@sads the discharge of the semen in the womb of a woman as a result of desires is considered as the first birth of man, and the birth of the son as the second birth and the birth elsewhere after death is regarded as the third birth. Thus it is said, "It is in man that there comes first the embryo, which is but the semen which is produced as the essence of all parts of his body and which holds itself within itself, and when it is put in a woman, that is his first birth. That embryo then becomes part of the woman's self like any part of her body; it therefore does not hurt her; she protects and develops the embryo within herself. As she protects (the embryo) so she also should be protected. It is the woman who bears the embryo (before birth) but when after birth the father takes care of the son always, he is taking care only of himself, for it is through sons alone that the continuity of the existence of people can be maintained. This is his second birth. He makes this self of his a representative for performing all the virtuous deeds. The other self of his after realizing himself and attaining age goes away and when going away he is born again that is his third birth" (Aitareya, II. 1-4) [Footnote ref 87]. No special emphasis is given in the Upani@sads to the sex-desire or the desire for a son; for, being called kama, whatever was the desire for a son was the same as the desire for money and the desire for money was the same as any other worldly desire (B@rh. IV. iv. 22), and hence sex-desires stand on the same plane as any other desire.