Sri Aurobindo translates Shwetashwatara Upanishad, Chapter Six, Verse 15: “One Swan of Being in the heart of all this Universe and He is Fire that lieth deep in the heart of water. By Knowledge of Him, the soul passeth beyond the pursuit of Death and there is no other road for the great passage.”
The Upanishads, which themselves indicate that they are the “secret of the Veda” frequently use symbolic imagery from the Veda to describe the experience of the Rishis. This verse has three such references, the “Swan of Being” and the “Fire” “in the heart of water.”
The Swan, the Fire and the Waters are all inextricably linked together in the Rig Veda. Sri Aurobindo comments on the symbolic nature of the Swan in The Secret of the Veda: “In the upward movement the horses that draw the chariot of the Ashwins change into birds, into swans, hamsasaha. The Bird in the Veda is the symbol, very frequently, of the soul liberated and upsoaring, at other times of energies so liberated and upsoaring, winging upwards towards the heights of our being, winging widely with a free flight, no longer involved in the ordinary limited movement or labouring gallop of the Life energy, the Horse, Ashwa. Such are the energies that draw the free car of the Lords of Delight, when there dawns on us the Sun of the Truth.” (pg. 320)
Traditional commentators have some difficulty making sense of this imagery when they try to fit it into a physical reality. How, for instance, do they make sense of the fire in the water? Modern science will tell you that water is a conductor of electricity, but that is not quite what the imagery conveys. It is only when the esoteric approach of spiritual development and experience is followed, that the Veda, and the Upanishads that rely on their imagery and symbolism, make complete sense and maintain a psychological inner consistency.
The water in the Rig Veda is the vast ocean of knowledge and the rivers are various forms of inspiration, whether inspired vision, inspired speech, inspired hearing, etc. The spiritual knowledge, based in the Oneness of all existence, and the controlling and all-inclusive nature of the Supreme, with omniscience, omnipotence, infinity, eternity and all-bliss as its characteristics, gives birth to the flame of aspiration and the powers of knowledge in the material plane, which is presided over by Agni Jatavedas, the “knower of all things born”, the Immortal in mortals, the flame that is born of the waters of this ocean of knowledge.
In The Secret of the Veda (pg. 108), Sri Aurobindo notes: “Obviously these are the waters of the Truth and the Bliss that flow from the supreme ocean. These rivers flow not upon earth, but in heaven. … They are, in other words, the waters of the vast Truth…. and (pg. 112) The waters in which the gods discovered the visible Agni cannot be terrestrial and material streams; this Agni who increases by knowledge and makes his home and rest in the source of the Truth, … who is increased by the divine waters in the unobstructed Vast, his own seat, and dwelling in that shoreless infinity yields to the illumined gods the supreme Immortality, cannot be the god of physical Fire..”
And on page 113: “Agni is the Deva, the All-Seer, manifested as conscious-force or, as it would be called in modern language, Divine or Cosmic Will, first hidden and building up the eternal worlds, then manifest, ‘born’, building up in man the Truth and the Immortality.”
Sri Aurobindo, The Upanishads, Shwetashwatara Upanishad, pp.369-384