The Bhagavad Gita introduces the concept of three modes or status-levels of the Self in order to aid the seeker in gaining insight and thereby gaining control over the lower nature during the development of the yogic action.
There are, in the Gita, the Kshara Purusha, the changing and mutable form that acts in the outer world. There is the Akshara Purusha, the unchanging, uninvolved Purusha that stands behind all action in the world without being affected. These are imaged in the Upanishads as the two birds sitting on a common tree, one of which eats the sweet fruit thereof, and the other sits by silently and observes. The Gita then posits yet a third Purusha, the Puroshottama, or the Supreme Self, which holds together, without contradiction, both the mutable and the immutable aspects, and exceeds them.
This view provides the leverage for the practitioner of Yoga to achieve psychological distance from the actions of the outer nature, and thereby liberate himself from bondage to the outer personality, and then subsequently make the step to integrate the outer with the inner life in a higher unity.
Sri Aurobindo describes the situation: “The mental being fixed in the Mutable moves in its flux and has not possession of an eternal peace and power and self-delight; the soul fixed in the Immutable holds all these in itself but cannot act in the world; but the soul that can live in the Highest enjoys the eternal peace and power and delight and wideness of being, is not bound in its self-knowledge and self-power by character and personality or by forms of its force and habits of its consciousness and yet uses them all with a large freedom and power for the self-expression of the Divine in the world. Here again the change is not any alteration of the essential modes of the Self, but consists in our emergence into the freedom of the Highest and the right use of the divine law of our being.”
Sri Aurobindo, The Synthesis of Yoga, Part Two: The Yoga of Integral Knowledge, Chapter 11, The Modes of the Self, pp. 362-363