Having granted the possibility of the survival of a highly developed mental and vital sheath or body from birth to birth, Sri Aurobindo points out that the physical body would still undergo dissolution and change due to its inability to adapt to the increasing requirements of the embodied Soul. “The physical being could only endure, if by some means its physical causes of decay and disruption could be overcome and at the same time it could be made so plastic and progressive in its structure and its functioning that it would answer to each change demanded of it by the progress of the inner person;(1) [(1) Even if science,–physical Science or occult Science,–were to discover the necessary conditions or means for an indefinite survival of the body, still, if the body could not adapt itself so as to become a fit instrument of expression for the inner growth, the soul would find some way to abandon it and pass on to a new incarnation. The material or physical causes of death are not its sole or its true cause; its true inmost reason is the spiritual necessity for the evolution of a new being.] it must be able to keep pace with the soul in its formation of self-expressive personality, its long unfolding of a secret spiritual divinity and the solow transformation of the mental into the divine mental or spiritual existence. This consummation of a triple immortality,–immortality of the nature completing the essential immortality of the Spirit and the psychic survival of death,–might be the crown of rebirth and a momentous indication of the conquest of the material Inconscience and Ignorance even in the very foundation of the reign of Matter. But the true immortality would still be the eternity of the Spirit; the physical survival could only be relative, terminable at will, a temporal sign of the Spirit’s victory here over Death and Matter.”
The essential difficulty of the physical body is not so much the breakdown of organ systems or functions, as we see constant progress on these levels to enable an extension and expansion of the normal productive life span; rather, the habitual reactions at the cellular level are an enormous limitation to the Soul’s need for change and evolutionary development. Unless the cellular habits can be overcome, the fulfillment of the conditions for physical Immortality can likely not be fruitfully achieved.
Sri Aurobindo, The Life Divine, Book 2, Part 2, Chapter 22, Rebirth and Other Worlds; Karma, the Soul and Immortality