“A man becomes good by good works, evil by evil.” Brihadaranyaka Upanishad. 3. 2.13
Brihadaranyaka Upanishad 4:4.3-7 continues giving more detailed concepts and goes on to explain the concept as follows:
“When a caterpillar has come to the end of a blade of grass, it reaches out to another blade, and draws itself over to it. In the same way the soul, having coming to the end of one life, reaches out to another body, and draws itself over to it.
“Just as a goldsmith, having taken a piece of gold, makes another form, new and more beautiful, so also, verily the Atman having cast off this body and having put away Avidya or ignorance, makes another new and more beautiful form” (Brihadaranyaka Upanishad).
“As people act, so they become. If their actions are good, they become good; if their actions are bad, they become bad. Good deeds purify those who perform them; bad deeds pollute those who perform them.
“Thus we may say that we are what we desire. Our will springs from our desires; our actions spring from our will; and what we are, springs from our actions. We may conclude, therefore, that the state of our desires at the time of death determines our next life; we return to earth in order to satisfy those desires.
“Consider those who in the course of many lives on earth have become free from desire. By this we mean that all their desires have found fulfillment within the soul itself. They do not die as others do. Since they understand God, they merge with God.
‘When all the desires clinging to the heart fall away, the mortal becomes immortal. When all the knots of desire strangling the heart are loosened, liberation occurs.
“As the snake discards its skin, leaving it lifeless on an anthill, so the soul free from desire discards the body, and unites with God who is eternal life and boundless light.”
Katha Upanishad 6.1-5, 10-11, 13-16
“The tree of eternity has its roots in the sky, and its branches reach down to earth. It is God; it is the immortal soul.
The whole universe comes from God; his energy burns like fire, and his power reverberates like thunder, in every part of the universe. In honor of God the sun shines, the clouds rain, and the winds blow. Death itself goes about its business in fear of God.
If you fail to see God in the present life, then after death you must take on another body; if you see God, then you will break free from the cycle of birth and death. God can be seen, like the reflection in a mirror, in a pure heart.
When the senses are calm and the mind is motionless, then your heart is pure; you have reached the highest state of consciousness, in which you are unified with God. If this state of consciousness is firm and secure, so it can never be broken, then you are free.
To calm the senses and still the mind, you must abandon the self. You must renounce ‘I’ and ‘me’ and ‘mine’. You must suppress every desire that surges around the heart. You must untie every knot of attachment.
A hundred and one lights radiate from the heart. One of them shines upwards to the crown of the head. This points the way to immortality. Every other light points to death.”
“Like corn, does a mortal ripen; like corn, does he spring to life again” (Kathopanishad).