The essentially human power of knowing, the reasoning intelligence, has its primary action in the organization, analysis and subsequent deductive reasoning processes that it applies to impressions of the senses, facts, perceived and delivered to its scrutiny by the sense organs. It operates most clearly on what appears to it to be external using this mechanism, although it may also turn its attention to the internal processes of the being, and even, with appropriate discipline, to its own action to test the validity and solidity of the conclusions it draws and the knowledge it acquires through that action.
Sri Aurobindo provides an extensive review of its operation: “The first business of the logical reason is therefore a right, careful and complete observation of its available material and data. The first and easiest field of data open to our knowledge is the world of Nature, of the physical objects made external to it by the separative action of mind, things not ourself and therefore only indirectly knowable by an interpreting of our sense perceptions, by observation, accumulated experience, inference and reflective thinking. Another field is our own internal being and its movements which one knows naturally by an internally acting mental sense, by intuitive perception and constant experience and by reflective though on the evidences of our nature. The reason with regard even to these inner movements acts best and knows the most correctly by detaching itself and regarding them quite impersonally and objectively, a movement which in the Yoga of knowledge ends in viewing our own active being too as not self, a mechanism of Nature like the rest of the world-existence. The knowledge of other thinking and conscious beings stands between these two fields, but is gained, too, indirectly by observation, by experience, by various means of communication and, acting on these, by reflection and inference largely founded on analogy from our knowledge of our own nature. Another field of data which the reason has to observe is its own action and the action of the whole human intelligence, for without that study it cannot be assured of the correctness of its knowledge or of right method and process.”
This field of activity of the human intelligence, however, does not encompass all of life and the universal manifestation, just as it does not encompass the causative and precedent Eternal and Infinite awareness of Sat-Chit-Ananda, which are clearly beyond the scope of the powers of the intellect. The mind may attempt to know the “unknowable” but comes back without attaining anything other than potentially a formation of thought or belief, a philosophy, religion or metaphysical exposition, but not the full reality. For those who are unable to even undertake this process, there is the common route of simply denying anything beyond what the reason can itself experience and understand.
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 23, The Supramental Instruments — Thought-process , pp. 820-821