Sri Aurobindo
Letters of Sri Aurobindo
Volume II - Part 3
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It is quite probable
that the sloka [Katha Upanishad 2.3.4] refers to a going up into higher worlds of felicity and light and this can be called a liberation or release. In later times the idea grew
strong that from all these higher worlds return is inevitable and it is only release
from all cosmic existence that gives mukti. The Vedic Rishis seem to have looked to an ascent into a divine luminous
world or state above the falsehood and ignorance. In the Upanishad the sun is the symbol of the supramental Truth and it is said that those who pass into it may return but those who pass through the gates of the Sun itself do not; possibly this means that an ascent into the supermind itself above the golden lid of overmind was the definitive liberation. The Veda speaks of the Truth hidden by a Truth where the Sun looses his horses from his car and there all the myriad rays are drawn together into One and that was considered the goal. The Isha Upanishad also speaks of the golden lid hiding the face of the Truth by removing which the Law of the Truth is seen and the highest 
knowledge in which the One Purusha is known (so’ham asmi) is described as the kalyāṇatama form of the Sun. All this seems to refer to the supramental states of which the Sun is the symbol.
Current publication:
Sri Aurobindo. Letters on Yoga. II // CWSA.- Volume 29. (≈ 22-24 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2013.- 522 p.
Other publications:
Sri Aurobindo. Letters on Yoga // SABCL.- Volume 22. (≈ 28 vol. of CWSA).- Pondicherry: Sri Aurobindo Ashram, 1971.- 502 p.
[Largest or earliest found passage: ] Sri Aurobindo. Letters of Sri Aurobindo: In 4 Series.- Second Series [On Yoga].- Bombay: Sri Aurobindo Sircle, 1949.- 599 p.