The necessity of rebirth hinges on the ultimate meaning of the manifestation of the universe. If it is random chance, or purely mechanical machinery, there is no necessity of rebirth. If, on the other hand, there is a conscious Will and Purpose to the universe, and the human being is part of that purpose and carrying out an evolutionary role, then rebirth becomes a centerpiece of the process.
Sri Aurobindo discusses these issues: “Once we find that there is a conscious Spirit of which this movement is one expression, or even admit that as our working hypothesis, we are bound to go on and ask whether this developing order ceases with what man now is or is laden with something more towards which it and he have to grow, an unfinished expression, a greater unfound term, and in that case it is evidently towards that greater thing that man must be growing; to prepare it and to realise it must be the stage beyond in his destiny. Towards that new step in the evolution his history as a race must be subconsciously tending and the powers of the highest individuals half consciently striving to be delivered of this greater birth; and since the ascending order of rebirth follows always the degrees of the evolution, that too cannot be meant to stop short or shoot off abruptly into the superconscient without any regard to the intended step.”
If we find that man is therefore a “transitional being”, as Sri Aurobindo has described elsewhere, then we can explore the “ladder of evolution” both with respect to the prior steps of the ascent of consciousness out of involvement in Matter, through the progressive development of Life, and the subsequent stages of expression of Mind, as well as the future stages beyond our current mental development into what Sri Aurobindo describes as the “supramental” ranges of consciousness.
“The whole processional significance of rebirth may be wrapped up in that one yet unattempted discovery.”