The life-mind of sensation, instinct and fulfillment of the desires is native to the animal consciousness and provides a first basis for the development of the human individual’s existence. With the development of the thinking mind, however, it no longer maintains the spontaneity and pure instinctive responses we find in the animal level of consciousness. It has been adapted and modified by the influence of the thinking mind. This provides both the advantages of the higher capacities of the mind, but at the same time reduces the purely effective responsiveness of the life-mind as we see it in its native action.
Sri Aurobindo notes” “It has been seized upon by the intelligence and profoundly altered by it even where the development of the intelligence is imperfect and itself most insistent in its prominence. It has lost most of its intuitive character, is indeed now infinitely richer as a supplier of materials and data, but no longer quite itself or at ease in its action because half rationalised, dependent at least on some infused element however vague of reasoning or intelligent activity and incapable of acting to good purpose without the aid of the intelligence. its roots and place of perfection are in the subconscient from which it emerges and man’s business is to increase in the sense of a more and more conscient knowledge and action. Man reverting to a governance of his being by the life-mind would become either irrational and erratic or dull and imbecile and would lose the essential character of manhood.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 23, The Supramental Instruments — Thought-process , pp. 818-819