It is the theory that the soul of a man is born again after his death. The पुराणस् and वेदस् contain many statements regarding this of which the following are a few:

What is Death?

When the जीवात्मा (soul) of an individual leaves his body with all its उपाधिस् (attributes and adjuncts) it is called Death. By उपाधिस् are meant the following four things. Mind and the senses.

The five प्राणस् namely प्राण , अपान , उदान , व्यान and समान . (iii) The सूक्ष्मशरीर , that is, the प्राणमनोविज्ञानकोशसङ्घात (the subtle body that is invisible with the grosser elements). (iv) कर्मन् (action). All these four things follow the soul even after his death. Only when the soul attains मोक्ष (salvation) do the उपाधिस् leave it. It is the life breath उदान that guides the soul out of the body. It is the सूक्ष्मशरीर that gives heat to the body while there is life and that is why when the soul leaves the body with the उपाधिस् the body becomes cold.

After death

When the soul leaves the body with the उपाधिस् it becomes active again and its activities and movements depend upon the actions of the soul while living. The spiritual actions of the individual are classified into three.

अपरब्रह्मोपासन

He who has done all his deeds according to scriptural injunctions is said to be one who has done उपासन of अपरब्रह्म . When such an individual dies his soul with all the उपाधिस् attains चन्द्र . He goes to चन्द्र through धूम (smoke), रात्रि (night), कृष्णपक्ष (the dark fortnight), दक्षिणायनकाल (the sun's passage south of the equator), पितृलोक (world of the manes) and आकाश (ether). He enjoys the rewards of the deeds done on earth there and reserving some to be enjoyed or suffered in his next birth the soul with the उपाधिस् comes back to earth to enter another body. The soul comes back to earth through ether, वायु , धूम , मेघ , वर्ष , व्रीहि , यव , औषधि , वृक्ष , तिल , पुरुषबीज and स्त्रीगर्भ . When the जीवात्मा goes to चन्द्र its पदार्थत्व (attribute) diminishes gradually and when it comes back to earth it increases gradually. Thus the जीवात्मा takes thousands of births going to and from the moon. The जीवात्मा coming back from the moon evolves from a plant to man. Plants are the food of man and the soul entering the plant enters the पुरुषबीज (semen virile) through food. The souls coming out as rebirths do accept wombs according to a definite principle. In the order of the merit of their good deeds on earth they are born in ब्रह्मिन् , क्षत्रिय , वैश्य or शूद्र wombs. The souls with great sins are born as worms and insects. This passage of the soul from earth to चन्द्र and vice versa is called पितृयान .

अपरविद्योपासन

There are some people who do not consider performance of religious rites as important but worship ब्रह्मन् . They do not treat ब्रह्मन् and जीवात्मा as one but view them as separate entities. Such devotees are called अपरविद्योपासकस् . Their souls as soon as they leave the body move towards अपरब्रह्मन् . The path to अपरब्रह्म is through अग्नि , ज्योतिस् , daytime, शुक्लपक्ष (the white fortnight), उत्तरायण (the passage of the sun to the north), samvatsara (year), सूर्य (sun), चन्द्र (moon) and विद्युत् (lightning). The dhanyas (blessed ones) who attain विद्युत् go to वरुणलोक , इन्द्रलोक and प्रजापतिलोक and then merge with परब्रह्म (the supreme being). This path is called देवयान . Since for both पितृयान and देवयान the जीवात्मा has to depend upon चन्द्र it is to be surmised that there is some special connection between जीवात्मा and चन्द्र . Those who attain अपरब्रह्म by the path of देवयान do not come back to earth. They have no rebirths. Those who have merged with अपरब्रह्म attain ब्रह्मन् by the end of a lifetime of ब्रह्मा . Thus those who attain ब्रह्मन् and do not believe that जीवात्मा and ब्रह्मन् are one and view them separately attain परब्रह्म through the merger with अपरब्रह्म . This is called क्रममुक्ति . Some sages are of opinion that those who attain अपरब्रह्म enjoy prosperity by mind. Those who live attached to worldly pleasures but not do things prohibited by the Vedas attain पितृलोक by the path of दक्षिणायन . After enjoying all the accrued 'puṇya' there, they come back again to earth to be born again.

पापोपासन

The जीवात्मा of one who does not follow the injunctions of शास्त्रस् correctly does not attain चन्द्रलोक . It is born again as pests and insects attaining a place called तृतीय . There is no evidence in the पुराणस् of their attaining मोक्ष . How a soul subjected to rebirths attains मोक्ष is described under 'Mokṣa'. (Chāndogya Upaniṣad, Gītā, Bhāgavata, Śvetāśvatara Upaniṣad) .