Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verses 14-15: “When every desire that finds lodging in the heart of man, has been loosened from its moorings, then this mortal puts on immortality: even here he tastes God, in this human body. Yea, when all the strings of the heart are rent asunder, even here, in this human birth, then the mortal becomes immortal. This is the whole teaching of the Scriptures.”
When we consider the term “immortality”, it becomes clear that there is considerable confusion in our minds. Some people equate the idea of immortality with “living forever” and by that, they mean in the body and personality that they currently identify with. They do not reflect on what this might mean, or how it might be brought about. They would obviously not expect to stay the same for all time, for if it occurred as a child, or as an aged person, they would not be happy with the result. Rather, they imagine immortality to mean a human being, in the prime of life, remaining that way continuously throughout time, living for years without end. The breakdown of the body is not anticipated. What if we lived forever in a body wracked with pain and suffering? No one would want to contemplate immortality that way!
Others conceive of immortality as something that occurs as a reward for their religious faith. They die and eventually will be resurrected, rejoined in their bodily life with their family, and live forever in heaven. Once again, they conceive of this as something that occurs to an identifable and specific personality with specific relationships. The mechanism has to be taken “on faith” as we may see the disintegration of the physical bodies in the earth and still believe that somehow the body will be brought back into its prime of life form upon the success of the resurrection.
Still others conceive of immortality differently and consider the physical life in the body to be something like a suit of clothes one puts on, and when it is worn or no longer useful, put off and exchanged for another. These individuals treat immortality as conscious serial births into new bodies, with a consistent individuality that experiences the changes.
What then do the sages of the Upanishads have to say about immortality. They equate immortality with overcoming the force of desire and abandoning attachment to the things of the world. For them, immortality is achieved in this human body through these methods. The sense is that it is the ego-consciousness that experiences birth, suffering, old age and death and a sense of mortality. When one has discovered the divine Self within, one is united in Oneness with the Eternal, and thereby participates in the universal manifestation from the divine standpoint, which does not concern itself with any individual personality and its perpetuation. Immortality is of the soul, not the body. There is a continuous stream of developing energy in the universe, the manifestation of the Divine, and to the extent the seeker identifies with this, not the individual ego, he experiences the true consciousness of the Divine which is immortal.
When we reflect on the various approaches to the aspiration of humanity for immortality, it becomes clear that the practical people of the world are unable to conceptualize it, while the Rishis and Sages, with their deep reflection and spiritual experience, have found a deep, true and essential meaning that fulfills the human aspiration for immortality. As the Isha Upanishad proclaims: “All this is for habitation by the Lord, whatsoever is individual universe of movement in the universal motion. By that renounced, thou shouldst enjoy; lust not after any man’s possession.”
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129