Sri Aurobindo
Letters on Yoga
5. Planes and Parts of the Being
I do not think exact correlations can always be traced between one system of spiritual and occult knowledge and another. All deal with the same material, but there are differences of standpoint, differences of view-range, a divergence in the mental idea of what is seen and experienced, disparate pragmatic purposes and therefore a difference in the paths surveyed, cut out or followed; the systems vary, each constructs its own schema and technique.
In the ancient Indian system there is only one triune supernal, Sachchidananda. Or if you speak of the upper hemisphere as the supernal, there are three, Sat plane, Chit plane and Ananda 
plane. The supermind could be added as a fourth, as it draws upon the other three and belongs to the upper hemisphere. The Indian systems did not distinguish between two quite different powers and levels of consciousness, one which we can call overmind and the other the true supermind or Divine Gnosis. That is the reason why they got confused about Maya (overmind-Force or Vidya-Avidya), and took it for the supreme creative power. In so stopping short at what was still a half-light they lost the secret of transformation – even though the Vaishnava and Tantra yogas groped to find it again and were sometimes on the verge of success. For the rest, this, I think, has been the stumbling-block of all attempts at the discovery of the dynamic divine Truth; I know of none that has not imagined, as soon as it felt the overmind lustres descending, that this was the true illumination, the Gnosis, with the result that they either stopped short there and could get no farther, or else concluded that this too was only Maya or Lila and that the one thing to do was to get beyond it into some immovable and inactive silence of the Supreme.
Perhaps, what may be meant by supernals is rather the three fundamentals of the present manifestation. In the Indian system, these are Ishwara, Shakti and Jiva, or else Sachchidananda, Maya and Jiva. But in our system which seeks to go beyond the present manifestation, these could very well be taken for granted and, looked at from the point of view of the planes of consciousness, the three highest – Ananda (with Sat and Chit resting upon it), supermind and overmind might be called the three Supernals. Overmind stands at the top of the lower hemisphere, and you have to pass through and beyond overmind, if you would reach supermind, while still above and beyond supermind are the worlds of Sachchidananda.
You speak of the gulf below the overmind. But is there a gulf – or any other gulf than human unconsciousness? In all the series of the planes or grades of consciousness there is nowhere any real gulf, always there are connecting gradations and one can ascend from step to step. Between the overmind and the human mind there are a number of more and more luminous gradations; but, as these are superconscient to human mind (except one or two of the lowest of which it gets some direct touches), it is apt to 
regard them as a superior Inconscience. So one of the Upanishads speaks of the Ishwara consciousness as suṣupti, deep Sleep, because it is only in Samadhi that man usually enters into it, so long as he does not try to turn his waking consciousness into a higher state.
There are in fact two systems simultaneously active in the organisation of the being and its parts: one is concentric, a series of rings or sheaths with the psychic at the centre; another is vertical, an ascension and descent, like a flight of steps, a series of superimposed planes with the supermind-overmind as the crucial nodus of the transition beyond the human into the Divine. For this transition, if it is to be at the same time a transformation, there is only one way, one path. First, there must be a conversion inwards, a going within to find the inmost psychic being and bring it out to the front, disclosing at the same time the inner mind, inner vital, inner physical parts of the nature. Next, there must be an ascension, a series of conversions upwards and a turning down to convert the lower parts. When one has made the inward conversion, one psychicises the whole lower nature so as to make it ready for the divine change. Going upwards, one passes beyond the human mind and at each stage of the ascent, there is a conversion into a new consciousness and an infusion of this new consciousness into the whole of the nature. Thus rising beyond intellect through illuminated higher mind to the intuitive consciousness, we begin to look at everything not from the intellect range or through intellect as an instrument, but from a greater intuitive height and through an intuitivised will, feeling, emotion, sensation and physical contact. So, proceeding from Intuition to a greater overmind height, there is a new conversion and we look at and experience everything from the overmind consciousness and through a mind, heart, vital and body surcharged with the overmind thought, sight, will, feeling, sensation, play of force and contact. But the last conversion is the supramental, for once there – once the nature is supramentalised, we are beyond the Ignorance and conversion of consciousness is no longer needed, though a farther divine progression, even an infinite development is still possible.
Current publication:
Sri Aurobindo. Letters on Yoga // SABCL.- Volume 22. (≈ 28 vol. of CWSA).- Pondicherry: Sri Aurobindo Ashram, 1971.- 502 p.
Other publications:
[Largest or earliest found passage: ] Sri Aurobindo. Letters on Himself and the Ashram // CWSA.- Volume 35. (≈ 26 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2011.- 658 p.
Sri Aurobindo. Letters on Yoga. I // CWSA.- Volume 28. (≈ 22 vol. of SABCL).- Pondicherry: Sri Aurobindo Ashram, 2012.- 590 p.
Sri Aurobindo. The Riddle of this World: Letters.- Pondicherry: Sri Aurobindo Ashram, 1973.- 72 p.