The development of the reflective power of Reason and instrumental power of the enlightened Will, is a great step forward in evolution embodied within the segment of humanity that has cultured these powers and focused on their implementation. The self-aware consciousness has the ability to distance itself, to a greater or lesser degree, from the habitual reactions and responses of the animal mentality that pervades most of our mental activity. Yet this action of consciousness, operative as the Buddhi, is clearly not the final stage or possibility for the development and action of consciousness, as it remains imperfect, limited and weakened in its action through the admixture of the physical and vital levels of consciousness, and through its own reliance on the mental principle, which is a principle of fragmentation and division in principle. What becomes obvious, however, is that this stage of development is a transitional stage and that further levels of consciousness are both necessary and possible of achievement.
Sri Aurobindo elaborates about the Buddhi: “Still it is a movement by which we come to the knowledge that there is a power within us greater than the animal life, a truth greater than the first truths or appearances perceived by the sense-mind, and can try to get at that truth and to labour towards a greater and more successful power of action and control, a more effective government both of our own nature and the nature of things around us, a higher knowledge, a higher power, a higher and larger enjoyment, a more exalted range of being. What then is the final object of this trend? Evidently, it must be for the Purusha to get to the highest and fullest truth of itself and of things, greatest truth of soul or self and greatest truth of Nature, and to an action and a status of being which shall be the result of or identical with that Truth, the power of this greatest knowledge and the enjoyment of that greatest being and consciousness to which it opens. This must be the final result of the evolution of the conscious being in Nature.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 7, Purification–Intelligence and Will, pp. 638-639