The Brahma Sutra, a Sanskrit text assigned to various dates between the 5th century BCE and the first Century CE, is one of the most complex texts, impossible to understand without a commentary. What, for instance, can the average reader understand from a one word sutra that says, ‘kampanat’, i.e, ‘trembles’. Only the commentators know what this refers to, and which passages in the Upanishads are connected with this.
After many passages explaining Brahman, the ultimate cause of the world and the only reality, the third chapter begins with a discussion on reincarnation. Quoting various Upanishads as usual, the commentators explain the terse short statements. When the individual soul departs from the body, they say, it is accompanied by subtle elements, as well as prana, or the breath, and by the eleven indriyas or senses. After spending some time in a heavenly world or in hell, the soul returns to earth to a new body, based on its residual karma, that is those actions that still have not been exhausted by enjoyment or sorrow in heaven or hell.
This brief account is of course, a simplification of the text, but provides some indications on theories of reincarnation.