With the development of the spiritual reason, a very substantial new power of knowing and acting becomes available to the evolving human being. It is easy to lead oneself to believe that this is the ultimate realisation of the supramental consciousness, yet Sri Aurobindo makes it clear that this is but a gradation on the way, not a final achievement in this regard.
The basic characteristics of mind are fragmentation, division and separation, which means that it is an instrument that starts in ignorance, tries to assemble facts and information and sense impressions and build them into some kind of coherent understanding of the world and its functioning. It has serious and insuperable limitations based on its experience of separation from the unity and oneness of the universe.
The spiritual reason gains access to the unity and oneness, but remains something of a reflected power rather than maintaining a native basis in that oneness. Sri Aurobindo notes: “At the same time the spiritual reason acts mainly by the representative idea and will in the spirit, though it has a greater and more essential truth as its constant source and supporter and reference. It is, then, a power of light of the Ishwara, but not the very self-power of his immediate presence in the being; it is his surya-shakti, not his whole atma-shakti or para sva prakrtih, that works in the spiritual reason.”
The action of the supermind in its native and complete power is something else: “The immediate self-power begins its direct operation in the greater supermind, and that takes up all that has hitherto been realised in body, life and mind and in the intuitive being and by the spiritual reason and shapes all that has been created, all that has been gathered, turned into stuff of experience and made part of the consciousness, personality and nature by the mental being, into a highest harmony with the high infinite and universal life of the spirit. ”
“The mind can have the touch of the infinite and the universal and can reflect and even lose itself in them, but the supermind alone can enable the individual to be completely one in action with the universal and transcendent Spirit.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 21, The Gradations of the Supermind, pp. 791-792