In order to gain an understanding of the differences between the mental consciousness and the supramental, it is necessary to first recognize the operations and characteristic action of the mental consciousness. The human mind, in its native operation, is represented as the mind of the reasoning intelligence. This power, however does not operate exclusively nor is it capable of managing the whole of the life of the individual. Below the operation of the reason, the mind of life, in addition to capturing sensations and delivering them to the mental apparatus, works through what we may call an instinctive process. Above the operation of the reason, there is an action of intuition, although this is quickly covered up by the mental process and thereby diluted or distorted in its effect.
Sri Aurobindo notes: “His mentality is inextricably subjected to a joint, mixed and intricate action of the reasoning intelligence with two other powers, an intuition, actually only half luminous in the human mentality, operating behind the more visible action of the reason or veiled and altered in the action of the normal intelligence, and the life-mind of sensation, instinct, impulse, which is in its own nature a sort of obscure involved intuition and which supplies the intelligence from below with its first materials and data. And each of these other powers is in its own kind an intimate action of the spirit operating in mind and life and has a more direct and spontaneous character and immediate power for perception and action than the reasoning intelligence. But yet neither of these powers is capable of organising for man his mental existence.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 23, The Supramental Instruments — Thought-process , pg. 818