Mundaka Upanishad, Chapter 3, Section 1, Verses 1 and 2, as translated by Sri Aurobindo: “Two birds, beautiful of wing, close companions, cling to one common tree: of the two one eats the sweet fruit of the tree, the other eats not but watches his fellow. The soul is the bird that sits immersed on the one common tree; but because he is not lord he is bewildered and has sorrow. But when he sees that other who is the Lord and beloved, he knows that all is His greatness and his sorrow passes away from him.”
The parable of the two birds is famous and appears in multiple Upanishads. (It appears also in the Shwetashwatara Upanishad for instance) The individual soul, the Jivatman, is focused on the life of the world, eating the sweet fruit of the tree, and because it is immersed in that life and the results of action in the world, it is subject to the illusion of separation and the suffering that attends it. The other bird represents the Divine consciousness which exists in a state of unity, not fragmentation. It witnesses the action of the fragmented consciousness of each individual, yet remains aloof and unmoved by the joy and suffering of the individuals. When the soul recognizes its oneness with the Divine, then it passes beyond sorrow and grief.
The parable illustrates the shift needed from the individual standpoint to the divine standpoint, and the result achieved thereby includes the release from suffering and the piercing through the veil of Maya and its illusion of separateness of the individual in the manifested world.
There is a practice of Yoga that involves the cultivation of the witness consciousness that observes the action of the body, life and mind of the individual. This practice helps bring about detachment and at the same time provides the seeker with a method to shift the standpoint to the divine standpoint. It leads to the awareness of the witness Purusha observing the active Prakriti. The witness sustains and supports but does not intervene in the action of Nature in the individual. As the practitioner becomes more successful in shifting to the witness consciousness, a new viewpoint arises which has its effects upon the action of Nature. As the Upanishad states “he knows that all is His greatness and his sorrow passes away from him.”
Sri Aurobindo, The Upanishads, Mundaka Upanishad, pp. 193-210