Sri Aurobindo translates Katha Upanishad, Second Cycle: Third Chapter, Verse 1: “Yama speaks: ‘This is an eternal Ashwattha-tree whose root is above, but its branches are downward. It is He that is called the Bright One and Brahman, and Immortality, and in Him are all the worlds established, none goes beyond Him. This is That thou seekest.’ “
The image of the Ashwattha-tree, representing the source and development of life, appears also in the Bhagavad Gita. The tree, known variously as the Peepul tree, the Bodhi Tree, the Ficus religiosa, is considered to be a sacred tree in India. The physical tree, of course, is rooted in the earth and expands upwards; but the Upanishad (and the Gita) reverse this normal order and have the roots above and the branches below. The source of all life is the Brahman and the world, and all its forms and beings, and forces, and developments grow and manifest from that source, thus, “the roots are above”.
The normal human viewpoint is rooted in the earth and believes everything starts with Matter. All life, mind and existence, from this viewpoint, somehow develops out of the primordial chemical soup of Matter. A shift in standpoint, however, shows us that Matter itself is not the primary or “first cause” and that there is something that precedes and originates material existence. This is the Brahman and this is the true “root of existence”.
Sri Aurobindo, The Upanishads, Katha Upanishad, pp. 213-241, and Kapali Sastry, Lights on the Upanishads, pp. 104-129