The changes required in the lower, outer nature are complicated by the various different aspects or processes that are not only independently active to carry out their respective functions, but are also interwoven with one another, take influences from each other and respond to the deformations that occur at other levels of the being. As the reasoning mind, and then the emotional mind become cleared of the greatest of the deformations and take on a purer form of action, there remains still the foundational mind of sensation and the mind of active impulse, the one receiving and sorting out the impulses and the other responding and sending out the reactions to the objects of the senses in the outer world.
Sri Aurobindo goes into greater detail on this subject: “The receptive sensational mind is the nervous mental basis of the affections; it receives mentally the impacts of things and gives to them the responses of mental pleasure and pain which are the starting-point of the duality of emotional liking and disliking. All the heart’s emotions have a corresponding nervous-mental accompaniment, and we often find that when the heart is freed of any will to the dualities, there still survives a root of disturbance of nervous mind, or a memory in physical mind which falls more and more away to a quite physical character, the more it is repelled by the will in the Buddhi. it becomes finally a mere suggestion from outside to which the nervous chords of the mind still occasionally respond until a complete purity liberates them into the same luminous universality of delight which the pure heart already possesses.”
A similar process needs to also take place with respect to the response given back through the organs of action to the impulses that come in. “The active dynamic mind of impulse is the lower organ or channel of responsive action; its deformation is a subjection to the suggestion of the impure emotional and sensational mentality and the desire of the Prana, to impulses to action dictated by grief, fear, hatred, desire, lust, craving, and the rest of the unquiet brood. Its right form of action is a pure dynamic force of strength, courage, temperamental power, not acting for itself or in obedience to the lower members, but as an impartial channel for the dictates of the pure intelligence and will or the supramental Purusha.”
“When we have got rid of these deformations and cleared the mentality for these truer forms of action, the lower mentality is purified and ready for perfection. But that perfection depends on the possession of a purified and enlightened Buddhi; for the Buddhi is the chief power in the mental being and the chief mental instrument of the Purusha.”
Sri Aurobindo, The Synthesis of Yoga, Part Four: The Yoga of Self-Perfection, Chapter 6, Purification–the Lower Mentality, pp. 634-635