The Buddhi, the conscious intelligence, represents a stage clearly beyond the mechanical workings of the sense mind and the nervous reactions to the physical and vital impulses that characterize most of the action of the mentality in the lower nature. it clearly cannot originate in the physical nature itself, but must represent some higher force of conscious awareness that can become active at a certain evolutionary stage. Along with the activation of the Buddhi there arises a heightened sense of self-awareness and the potential for a reflective consciousness that is not entirely fixated on the sense-impressions that impinge on the mentality from the outer world. While we see the rudiments of a consciousness of self in the higher animal kingdom, and a self-preservation instinct throughout vital nature, the development of a self-aware ego-sense comes with the development of this higher mentality as expressed in the Buddhi.
Sri Aurobindo observes: “Buddhi is really an intermediary between a much higher Truth-mind not now in our active possession, which is the direct instrument of Spirit, and the physical life of the human mind evolved in body. Its powers of intelligence and will are drawn from this greater direct Truth-mind or supermind. Buddhi centres its mental action round the ego-idea, the idea that I am this mind, life and body or am a mental being determined by their action. It serves this ego-idea whether limited by what we call egoism or extended by sympathy with the life around us. An ego-sense is created which reposes on the separative action of the body, of the individualised life, of the mind-responses, and the ego-idea in the Buddhi centralises the whole action of this ego’s thought, character, personality.”
The higher reasoning faculty which we call Buddhi can be turned downwards and outwards to bring a new level of understanding and power of action to the individual, but it also has the capability to turn its light inward and upward to the higher levels of spiritual awareness: “But when the highest reason and will develop, we can turn towards that which these outward things mean to the higher spiritual consciousness. The ‘I’ can then be seen as a mental reflection of the Self, the Spirit, the Divine, the one existence transcendent, universal, individual in its multiplicity; the consciousness in which these things meet, become aspects of one being and assume their right relations, can then be unveiled out of all these physical and mental coverings. When the transition to supermind takes place, the powers of the Buddhi do not perish, but have all to be converted to their supramental values.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 5, The Instruments of the Spirit, pp. 625-626