Sri Aurobindo
Letters of Sri Aurobindo
Volume I - Part 3
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What is meant here1 is the Divine in its essential manifestation which reveals itself to us as Light and Consciousness, Power, Love and Beauty. But in its actual cosmic manifestation the Supreme, being the Infinite and not bound by any limitation, can manifest in itself, in its consciousness of innumerable possibilities, something that seems to be the opposite of itself, something in which there can be Darkness, Inconscience, Inertia, Insensibility, Disharmony and Disintegration. It is this that we see at the basis of the material world and speak of nowadays as the Inconscient – the inconscient Ocean of the Rigveda in which the One was hidden and arose in the form of this Universe,– 
or, as it is sometimes called, the non-being, Asat. The Ignorance which is the characteristic of our mind and life is the result of this origin in the Inconscience. Moreover, in the evolution out of inconscient existence there rise up naturally powers and beings which are interested in the maintenance of all negations of the Divine, error and unconsciousness, pain, suffering, obscurity, death, weakness, illness, disharmony, evil. Hence the perversion of the manifestation here, its inability to reveal the true essence of the Divine. Yet in the
very base of this evolution all that is divine is there involved and pressing to evolve, Light, Consciousness, Power, Perfection, Beauty, Love. For in the Inconscient itself and behind the perversions of the Ignorance Divine
Consciousness lies concealed and works and must more and more appear, throwing off in the end its disguises. That is why it is said that the world is called to express the Divine.
Your statement about the supramental evolution is correct except that it does not follow that humanity as a whole will become supramental. What is more likely to happen is that the supramental principle will be established in the evolution by the descent just as the mental principle was established by the appearance of thinking Mind and Man in earthly life. There will be a race of supramental beings on the earth just as now there is a race of mental beings. Man himself will find a greater possibility of rising to the planes intermediary between his mind and supermind and making their powers effective in his life, which will mean a great change in humanity on earth, but it is not likely that the mental stage will disappear from the ascending ladder and, if so, the continued existence of a mental race will be necessary so as to form a stage between the vital and the supramental in the evolutionary movement of the spirit.
Such a descent of higher beings as you suggest may be envisaged as a part of the process of the change. But the main part of the change will be the appearance of the supramental being and the organisation of a supramental nature here, as a mental being has appeared and a mental nature organised itself during the last stage of the evolution. I prefer nowadays not to speak of the descent of the higher beings because my experience is that 
it leads in the minds of the sadhaks to
a vain and often egoistic romanticism which distracts the attention from the real work, that of the realisation of the Divine and the transformation of the nature.
Current publication:
Sri Aurobindo. Letters on Yoga. I // The Complete Works of Sri Aurobindo. Set of 37 Volumes (CWSA).- Volume 28. (≈ 22 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2012.- 590 p.
Other publications:
Sri Aurobindo. Letters on Yoga // Sri Aurobindo Birth Century Library. Set in 30 Volumes (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.- First Series [On Yoga].- Bombay: Sri Aurobindo Sircle, 1947.- 416 p.