The Kaṭha Upaniṣad states that one bird on the tree of the body is eating the fruits, and the other bird is simply witnessing. The roots of this tree extend in three directions. This means that the root of the tree is the three modes of material nature: goodness, passion and ignorance. Just as the tree’s root expands, so, by association of the modes of material nature (goodness, passion and ignorance), one expands his duration of material existence. The tastes of the fruits are of four kinds: religiosity, economic development, sense gratification and, ultimately, liberation. According to the different associations in the three modes of material nature, the living entities are tasting different kinds of religiosity, different kinds of economic development, different kinds of sense gratification and different kinds of liberation. Practically all material work is performed in ignorance, but because there are three qualities, sometimes the quality of ignorance is covered with goodness or passion. The taste of these material fruits is accepted through five senses. The five sense organs through which knowledge is acquired are subjected to six kinds of whips: lamentation, illusion, infirmity, death, hunger and thirst. This material body, or the material manifestation, is covered by seven layers: skin, flesh, blood, marrow, bone, fat and semen. The branches of the tree are eight: earth, water, fire, air, ether, mind, intelligence and ego. There are nine gates in this body: the two eyes, two nostrils, two ears, one mouth, one genital, one rectum. And there are ten kinds of internal air passing within the body: prāṇa, apāna, udāna, vyāna, samāna, etc. The two birds seated in this tree, as explained above, are the living entity and the localized Supreme Personality of Godhead, Paramātmā.
The root cause of the material manifestation described here is the Supreme Personality of Godhead. The Supreme Personality of Godhead expands Himself to take charge of the three qualities of the material world. Viṣṇu takes charge of the mode of goodness, Brahmā takes charge of the mode of passion, and Lord Śiva takes charge of the mode of ignorance. Brahmā, by the mode of passion, creates this manifestation, Lord Viṣṇu maintains this manifestation by the mode of goodness, and Lord Śiva annihilates it by the mode of ignorance. The whole creation ultimately rests in the Supreme Lord. He is the cause of creation, maintenance and dissolution. And when the whole manifestation is dissolved, in its subtle form as the Supreme Lord’s energy it rests within His body.
“At present,” the demigods prayed, “the Supreme Lord Kṛṣṇa is appearing just for the maintenance of this manifestation. Actually the Supreme Cause is one, but less intelligent persons, being deluded by the three modes of material nature, see that the material world is manifested through different causes. Those who are intelligent can see that the cause is one, Kṛṣṇa.” As it is stated in the Brahma-saṁhitā, īśvaraḥ paramaḥ krsnah . . . sarva-kāraṇa-kāraṇam (BS 5.1). “Kṛṣṇa, the Supreme Personality of Godhead, is the cause of all causes.” Brahmā is the deputed agent for creation, Viṣṇu is the expansion of Kṛṣṇa for maintenance, and Lord Śiva is the expansion of Kṛṣṇa for dissolution.