The ego, as we will recall from our review of the principles of existence enunciated by Sankhya, is part of the machinery of Nature, and as such, it is subject to the action of the three Gunas, or modes, of Nature. Because of our identification with the ego in the normal human consciousness, we identify with the actions of Nature within us and are constantly driven by the force of desire which arises through the action of the Gunas.
The true soul is separate from the actions of Nature in its essence, it is associated with the concept of the Purusha. The Purusha is the witness, Lord and Master of Nature, and is unattached and unaffected by the actions of Nature. The Upanishadic verse that describes, two birds which sit on a common tree, one watching and sanctioning, while the other eats of the fruit of the tree, is a poetic description of the difference between the true soul and the apparent self that is part of the mechanism of Nature.
Sri Aurobindo describes this further: “It is really the ego which is subject to Nature, inevitably, because it is itself part of Nature, one functioning of her machinery; but when the self-awareness in the mind-consciousness identifies itself with the ego, it creates the appearance of a lower self, an ego-self. And so too what we think of ordinarily as the soul is really the natural personality, not the true Person, the Purusha, but the desire-soul in us which is a reflection of the consciousness of the Purusha in the workings of Prakriti: it is, in fact, itself only an action of the three modes and therefore a part of Nature. Thus there are, we may say, two souls in us, the apparent or desire-soul, which changes with the mutations of the Gunas and is entirely constituted and determined by them, and the free and eternal Purusha not limited by Nature and her Gunas. We have two selves, the apparent self, which is only the ego, that mental centre in us which takes up this mutable action of Prakriti, this mutable personality….and the true self which is, indeed, the upholder, the possessor and the lord of Nature and figured in her, but is not itself the mutable natural personality.”
Sri Aurobindo, Essays on the Gita, First Series, Chapter 21, The Determinism of Nature, pp. 204-205