The principle of the mental action is to divide, to analyse, to segment and measure. This characteristic remains active up through the overmental ranges of consciousness until the Supermind has been reached, according to Sri Aurobindo. The Supermind is the level that fits all the segmented parts together into a unified whole, and which can see both the divisions and the unity in one view. The mental action, due to its inherent divided nature and its limitations of both scope and power, tends to seek after one line of solution at the expense of others. When the overmental consciousness begins to descend and act, it is able to link together these varying mental solutions in a more embracing harmony, but it is restricted and limited by the necessity of acting within the framework of mind, life and body.
Sri Aurobindo points out that this necessitates the third transformation to complete the two prior ones: “As the psychic change has to call in the spiritual to complete it, so the first spiritual change has to call in the supramental transformation to complete it. For all these steps forward are, like those before them, transition; the whole radical change in the evolution from a basis of Ignorance to a basis of Knowledge can only come by the intervention of the supramental Power and its direct action in earth-existence.”
He makes it clear that it is the supramental consciousness that completes the transformative action: “Only the Supermind can thus descend without losing its full power of action; for its action is always intrinsic and automatic, its will and knowledge identical and the result commensurate: its nature is a self-achieving Truth-Consciousness and, if it limits itself or its working, it is by choice and intention, not by compulsion; in the limits it chooses its action and the results of its action are harmonious and inevitable.”
Sri Aurobindo concludes: “This then must be the nature of the third and final transformation which finishes the passage of the soul through the Ignorance and bases its consciousness, its life, its power and form of manifestation on a complete and completely effective self-knowledge. The Truth-Consciousness, finding evolutionary Nature ready, has to descend into her and enable her to liberate the supramental principle within her; so must be created the supramental and spiritual being as the first unveiled manifestation of the truth of the Self and Spirit in the material universe.”
Sri Aurobindo, The Life Divine, Book 2, Part 2, Chapter 25, “The Triple Transformation”