The distinction between the Purusha, the unchanging witness consciousness, and Prakriti, the mutable nature, is useful to help us distinguish the complex interactions between the mental being, which has its basis in the Purusha, and the ego-personality that goes through changes of time, space and circumstance, and which is based in the Prakriti. This ego-personality is in fact made up of a number of different parts or layers that interact with each other and may in fact not always agree with one another in terms of the focus or direction.
With this basis of understanding, we can then look at the process of the mental being forming and developing a specific combination to create a new ego-personality in a new birth. Sri Aurobindo describes this process: “The mental being in resuming bodily life forms a new personality for its new terrestrial existence; it takes material from the common matter-stuff, life-stuff, mind-stuff of the physical world and during earthly life it is constantly absorbing fresh material, throwing out what is used up, changing its bodily, nervous and mental tissues. But this is all surface work; behind is the foundation of past experience held back from the physical memory so that the superficial consciousness may not be troubled or interfered with by the conscious burden of the past but may concentrate on the work immediately in hand. Still that foundation of past experience is the bed-rock of personality; and it is more than that. It is our real fund on which we can always draw even apart from our present superficial commerce with our surroundings. That commerce adds to our gains, modifies the foundation for a subsequent existence.”
There is of course much more behind this surface interaction and ego-personality. This adds further layers of complexity that must be reviewed and understood to get a more complete picture of what is really taking place when we speak of “rebirth” and “karma”.