In exploring Advaita, we may have heard of the metaphor of the two birds,

Two birds, inseparable friends, cling to the same tree. One of them eats the sweet fruit, the other looks on without eating. [1]

The two birds are the jiva (the one which eats) and paramatman (the one which looks on without eating). The jiva is bound, attached to karma and its fruits, whereas the paramatman is free from karma. Identified as the jiva, the ‘enjoyer’, we ‘taste’ the fruits of action (pleasure and pain). Identified with the paramatman, we do not experience the duality of pleasure and pain as there is no attachment to them.

The two birds highlight the contrasting ways of conducting action – with or without attachment. In the jiva, we act to attain certain fruits (desirable outcomes) of our actions. Whereas, in the paramatman, we act without any desire or discrimination between success and failure or pleasure and pain. The paramatman is the Advaitin witness, whilst the jiva is still caught up in the dualistic experience of self (subject) and ‘other’ (object).

What we may not know is that the ‘two birds’ metaphor originates from Rig Veda (1.164.20). It later features in the Mundaka (3.1.1) and Shvetashvatara Upanishads (4.6), where its meaning is explained,

On the same tree a person sits grieving, drowned (in sorrow), bewildered, feeling helpless. When they see the other, powerful Lord, content, sees his greatness, they are freed from suffering. When the seer sees the brilliant maker and the Lord as the Self who has their source in Brahman, then they are wise, they shake off good and evil. Stainless, they reach the highest equanimity (Mundaka 3.1.2-3 translated by Roebuck).

As the jiva, it is we who eat the fruit, ensnared in bondage, attached to pleasure and desire, whilst the “other”, paramatman, here called “Lord” (Isha), is free. When we realise the paramatman as, “the Self who has their source in Brahman”, we are freed from suffering and, all duality, including that of ‘two birds’, falls away. In the tree of the body hangs the ‘fruits’ of karma (attached action) of all living beings. As Shankara identifies, the tree is the body and the jiva and paramatman are said to reside in the ‘same tree’ because the place where they can both be perceived is identical or ‘inseparable’ [2].

The ‘shaking off’ is the burning of karma accrued from both good and evil actions. One is ‘stainless’ or pure as there is no longer the false impression of being a separate jiva  which leads to the production of karma. When we fail to see Brahman as the “source”, we mistake its forms for being separate, creating the experience as the jiva. The opposite, reaching the “highest equanimity”, is seeing that all is Brahman. As there is only Brahman – “one alone without a second” (Chandogya 6.8.21) – no separation or duality really exists and the jiva is illusory [3].

[1] Translated by Müller in The Upanishads. 
[2] Shankara’s Bhashya Mundaka 3.1.1
[3] Shankara’s Bhashya Mundaka 3.1.2 similarly declares that one who knows this thinks, 

“I am the atman, alike in all, seated in every living thing and not the other, illusory atman (jiva) enclosed under conditions created by ignorance, and this glory, this universe, is mine, the lord of all,” then he becomes absolved from grief (translated by S. Sitarama Sastri).