Sri Aurobindo translates Katha Upanishad First Cycle, First Chapter, Verses 12 and 13: “Nachiketas speaks: ‘In heaven fear is not at all, in heaven, O Death, thou art not, nor old age and its terrors; crossing over hunger and thirst as over two rivers, leaving sorrow behind the soul in heaven rejoices. Therefore that heavenly Flame (1) which thou, O Death, studiest, expound unto me, for I believe. They who win their world of heaven, have immortality for their portion. This for the second boon I have chosen.’ ”
(1) “The celestial force concealed subconsciently in man’s mortality by the kindling of which and its right ordering man transcends his earthly nature; not the physical flame of the external sacrifice to which these profound phrases are inapplicable.”
The first boon established the connection between the physical life in the world and the spiritual realms of existence, and provided the seeker the power to cross the barrier and then return with the knowledge to be obtained there. The next step, therefore, is for the seeker to understand the celestial fire, svargya agni. Agni is called the “knower of all things born”, so knowledge of this fire and its action in the universe brings right knowledge of the manifestation and the human being’s role in it. This knowledge is not obtained with a skeptical mind, and thus, there is the precondition of faith, as Nachiketas affirms “for I believe”. The knowledge of this fire conveys a true understanding of immortality, passing beyond death, and transcendence over hunger, thirst and sorrow, old age and death.
The Buddha formulated the four noble truths and sought for the solution to escape the grasp of sorrow, sickness, old age and death. This Upanishad indicates that it is the knowledge of this celestial fire that will be the key that the Buddha sought to overcome these causes of suffering.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129