The Taittiriya Upanishad relates the realization of Bhrigu when asked to concentrate: “Seek thou to know that from which these creatures are born, whereby being born they live and to which they go hence and enter again; for that is the Eternal.” (Sri Aurobindo, The Upanishads, Taittiriya Upanishad, Bhriguvalli, Chapter 1, pg. 275)
As a result of that concentration, the seeker first came upon Matter as the cause. With further concentration, he recognized the Vital Force. This too he overpassed eventually and came to know Mind as the source. The process continued thereafter. What is interesting however is the progression which mirrors the planes or sheaths that make up the life in the world, each one successively more subtle, larger, more flexible and more powerful as a creative force than the preceding one, and each one exercising its influence on the previous ones and their creation, existence and modifications.
Sri Aurobindo observes that beyond the physical worlds and vital worlds there are also mental worlds, where the operative principle is that of Mind. “Mind there is not determined by material conditions or by the life-force, but itself determines and uses them for its own satisfaction. There mind, that is to say, the psychical and intellectual being, is free in a certain sense, free at least to satisfy and fulfil itself in a way hardly conceivable to our body-bound and life-bound mentality; for the Purusha there is the pure mental being and his relations with Prakriti are determined by that purer mentality, Nature there is mental rather than vital and physical. Both the life-world and indirectly the material are a projection from that, the result of certain tendencies of the mental Being which have sought a field, conditions, an arrangement of harmonies proper to themselves; and the phenomena of mind in this world may be said to be a result of the pressure of that plane first on the life-world and then on life in the material existence. By its modification in the life-world it creates in us the desire-mind; in its own right it awakes in us the purer powers of our psychical and intellectual existence. But our surface mentality is only a secondary result of a larger subliminal mentality whose proper seat is the mental plane. This world of mental existence also is constantly acting upon us and our world, has its powers and its beings, is related to us through our mental body. There we find the psychical and mental heavens to which the Purusha can ascend when it drops this physical body and can there sojourn till the impulse to terrestrial existence again draws it downward. Here too are many planes, the lowest converging upon and melting into the worlds below, the highest at the heights of the mind-power into the worlds of a more spiritual existence.”
Sri Aurobindo, The Synthesis of Yoga, Part Two: The Yoga of Integral Knowledge, Chapter 19, The Planes of Our Existence, pp. 435-436