To understand the triple poise of the Soul, the Purusha, as described by the Gita, it is essential to understand the terminology used by the Gita. The aspect of the Soul involved in the play of Nature and immersed in it is called the Kshara Purusha, the “mutable”, as it takes on the color of the changes of Nature. The aspect of the Soul that is silent, immobile and detached from the play of Nature is called the Akshara Purusha, the “immutable”. The Upanishadic verse about two birds sitting on a common tree, one of them eating the sweet fruit thereof, and the other observing, is a clear reference to these two aspects of the Soul.
Sri Aurobindo describes these two aspects: “In the Kshara the Soul is involved in the actions of Nature, therefore it is concentrated, loses itself, as it were, in the moments of Time, in the waves of the Becoming, not really, but only in appearance and by following the current; in the Akshara Nature falls to silence and rest in the Soul, therefore it becomes aware of its immutable Being. The Kshara is the Sankhya’s Purusha when it reflects the varied workings of the Gunas of Nature, and it knows itself as the Saguna, the Personal; the Akshara is the Sankhya’s Purusha when these Gunas have fallen into a state of equilibrium, and it knows itself as the Nirguna, the Impersonal.”
“The Soul of man, when it takes the poise of the Kshara, identifies itself with the play of personality and readily clouds its self-knowledge with the ego-sense in Nature, so that he thinks of himself as the ego-doer of works; when it takes its poise in the Akshara, it identifies itself with the Impersonal and is aware of Nature as the doer and itself as the inactive witnessing Self….”
“The mind of man has to tend to one of these poises, it takes them as alternatives; it is bound by Nature to action in the mutations of quality and personality or it is free from her workings in immutable personality.”
Of course, this is as long as we remain bound in the mental framework of duality, as the Gita proposes an integrating poise, the Purushottama, that holds both Kshara and Akshara as aspects but not irreconcilable opposites.
Sri Aurobindo, Essays on the Gita, First Series, Chapter 22, Beyond the Modes of Nature, pg. 219