Rene Descartes, the famous French philosopher, once declared “I think, therefore I am.” While he may not have captured the complete proof of his existence, he did highlight the unique role that the reasoning intelligence plays for the human being. When we observe the world around us, the primary quality that sets the human being apart from the rest of the creation is the self-aware, reasoning mind which has begun to manifest and provide a character to the human experience that is not found generally in the animal kingdom. It is therefore both reasonable and expected that it will be the purification of the reasoning intelligence that acts as the lynch-pin to the purification and transformation of the rest of the nature.
Sri Aurobindo notes: “Pending the evolution of any higher supramental power the intelligent will must be our main force for effectuation and to purify it becomes a very primary necessity. Once our intelligence and will are well purified of all that limits them and gives them a wrong action or wrong direction, they can easily be perfected, can be made to responde to the suggestions of Truth, understand themselves and the rest of the being, see clearly and with a fine and scrupulous accuracy what they are doing and follow out the right way to do it without any hesitating or eager error or stumbling deviation.”
Another benefit of this process in the Buddhi is that it can open the way and make possible the influence of the higher levels of insight, intuition and knowledge that are native to higher planes but which can influence the receptive and purified intelligent mind.
The primary obstacle, in Sri Aurobindo’s analysis, to the purification of the Buddhi is its interaction with the vital principle in the form of the psychic Prana: “But this purification cannot be effected without a preliminary clearing of its natural obstacles in the other lower parts of the antahkarana, and the chief natural obstacle running through the whole action of the antahkarana, through the sense, the mental sensation, emotion, dynamic impulse, intelligence, will, is the intermiscence and the compelling claim of the psychic Prana. This then must be dealt with, its dominating intermiscence ruled out, its claim denied, itself quieted and prepared for purification.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 6, Purification–the Lower Mentality, pp. 627-628