The highest and purest form of action of the Buddhi, the reasoning intelligence, comes when it separates itself from the lower actions of the sense-mind, the interface with the sense-mind or even the pragmatic reasoning intelligence that can be seen in earlier stages of its development. The reasoning mind is capable of abstract conceptualisation. It can focus on principles of knowledge and understanding that are not tied to personal gain, desire, or specific results or benefits to be obtained in the world of life and action. In its focus on the abstract knowledge, it is possible for it to lose its grounding in the world of life and action, and this can be chalked up as one of the limitations of the mentality at its highest level: it is only at a still higher level, beyond the farthest reaches of the mind, that this higher form of integration of knowledge and will can actually take place.
Sri Aurobindo observes: “The third and noblest stage of the intellectual will and reason is an intelligence which seeks for some universal reality or from a still higher self-existent Truth for its own sake and tries to live in that Truth. This is primarily a mind of knowledge and only secondarily a mind of Will.”
When it tries to act, it is forced to rely on the pragmatic intelligence, and this brings about a mixed and diluted action, and thus, a flawed implementation of the Truth as seen by this higher stage of Reason. “The disparity, even when it is not an opposition, between knowledge and will is one of the principal defects of the human Buddhi.”
As long as the individual remains embodied, of course, there cannot be any absolute purity or separate function of the Buddhi. There are always limitations, admixtures and imperfect data that distort the functioning of the reason. “Purified as much as may be from that habit of mental degradation, the human Buddhi is still a power that searches for the Truth, but is never in full or direct possession of it; it can only reflect truth of the spirit and try to make it its own by giving it a limited mental value and a distinct mental body.”
It also is limited by the partiality caused by the fragmented view and understanding of the human mentality and this limitation of viewpoint also tends to create fixed opinion rather than a wider, all-encompassing vision of the truth of things in their totality. This leads to preconception and bias in terms of how the truth of life is to be understood. “Release it as much as possible from this habit of limiting opinion, still it is subject to another affliction, the demand of the pragmatic mind for immediate effectuation, which gives it no time to proceed to large truth, but fixes it by the power of effective realisation in whatever it has already judged, known and lived. Freed from all these chains, the Buddhi can become a pure and flexible reflector of Truth, adding light to light, proceeding from realisation to realisation. It is then limited only by its own inherent limitations.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 7, Purification–Intelligence and Will, pp. 644-645