Sri Aurobindo
Letters of Sri Aurobindo
8. Rebirth
1. The psychic being stands behind mind, life and body, supporting them; so also the psychic world is not one world in the scale like the mental, vital or physical worlds, but stands behind all these and it is there that the souls evolving here retire for the time between life and life. If the psychic were only one principle 
in the rising order of body, life and mind on a par with the others and placed somewhere in the scale on the same footing as the others, it could not be the soul of all the rest, the divine element making the evolution of the others possible and using them as instruments for a growth through cosmic experience towards the Divine. So also the psychic world cannot be one among the other worlds to which the evolutionary being goes for supraphysical experience; it is a plane where it retires into itself for rest, for a spiritual assimilation of what it has experienced and for a replunging into its own fundamental consciousness and psychic nature.
2. For the few who go out of the Ignorance and enter into Nirvana, there is no question of their going straight up into higher worlds of manifestation. Nirvana or Moksha is a liberated condition of the being, not a world – it is a withdrawal from the worlds and the manifestation. The analogy of pitṛyāna and devayāna can hardly be mentioned in this connection.
3. The condition of the souls that retire into the psychic world is entirely static; each withdraws into himself and is not interacting with the others. When they come out of their trance, they are ready to go down into a new life, but meanwhile they do not act upon the earth life. There are other beings, guardians of the psychic world, but they are concerned only with the psychic world itself and the return of the souls to reincarnation, not with the earth.
4. A being of a psychic world cannot get fused into the soul of a human being on earth. What happens in some cases is that a very advanced psychic being sometimes sends down an emanation which resides in a human being and prepares it until it is ready for the psychic being itself to enter into the life. This happens when some special work has to be done and the human vehicle prepared. Such a descent produces a remarkable change of a sudden character in the personality and the nature.
5. Usually, a soul follows continuously the same line of sex. If there are shiftings of sex, it is, as a rule, a matter of parts of the personality which are not central.
6. As regards the stage at which the soul returning for rebirth enters the new body no rule can be laid down, for the 
circumstances vary with the individual. Some psychic beings get into relation with the birth-environment and the parents from the time of conception and determine the preparation of the personality and future in the embryo, others join only at the time of delivery, others even later on in the life and in these cases it is some emanation of the psychic being which upholds the life. It should be noted that the conditions of the future birth are determined fundamentally not during the stay in the psychic world but at the time of death – the psychic being then chooses what it should work out in the next terrestrial appearance and the conditions arrange themselves accordingly.
Note that the idea of rebirth and the circumstances of the new life as a reward or punishment of puṇya or pāpa is a crude human idea of “justice” which is quite unphilosophical and unspiritual and distorts the true intention of life. Life here is an evolution and the soul grows by experience, working out by it this or that in the nature, and if there is suffering, it is for the purpose of that working out, not as a judgment inflicted by God or Cosmic Law on the errors or stumblings which are inevitable in the Ignorance.