While the pure action of the Buddhi is clearly much more powerful and capable of receiving and responding to a greater Truth, it is still bound by the limitations of the mentality. It is a mental power and inherently incapable of grasping and holding the completeness and complexity of existence within its compass. Realisations gained on the mental level do not necessarily translate into the entire being and life, and in fact, they are always hampered by the fragmentation inherent in the mental being.
Sri Aurobindo notes: “…to get to the Truth itself we have to go beyond the mental Buddhi. Again, the nature of the mind prevents it from making an effective unification of the truths it seizes. it can only put them side by side and see oppositions or effect some kind of partial, executive and practical combination. But it finds finally that the aspects of the Truth are infinite and that none of its intellectual forms are quite valid, because the spirit is infinite and in the spirit all is true, but nothing in the mind can give the whole truth of the spirit.”
These limitations tend to make the Buddhi quite incapable of effective implementation of the truths it perceives because it cannot integrate them into a unified whole and cannot express them in a complete and unified manner when it comes time to act upon them. Everything is therefore somewhat fragmented and partial when it is expressed through the mentality. “It acts in a helpless limitation of Ignorance, though it may hold a Truth far greater than its action.”
Faced with these limitations, the being may simply turn away from the difficulties and complexities of carrying out the higher truth in life and focus its efforts on overpassing the mental limitations through a one-pointed focus on what is Beyond. “This it may do by seizing on some aspect, some principle, some symbol or suggestion of reality and pushing that to its absolute, all-absorbing, all-excluding term of realisation or by seizing on and realising some idea of indeterminate Being or Non-Being from which all thought and life fall away into cessation. The Buddhi casts itself into a luminous sleep and the soul passes away into some ineffable height of spiritual being.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 7, Purification–Intelligence and Will, pg. 645