The great fundamental doctrine of the Upanishads is the identity of the individual ātman with the world Ātman. It is most forcibly expressed in a frequently repeated sentence of the Chhāndogya Upanishad (vi. 8–16): “This whole world consists of it: that is the Real, that is the Soul, that art thou, O Çvetaketu.” In that famous formula, “That art thou” (tat tvam asi), all the teachings of the Upanishads are summed up. The Bṛihadāraṇyaka (I. iv. 6) expresses the same doctrine thus: “Whoever knows this, ‘I am brahma’ (aham brahma asmi), becomes the All. Even the gods are not able to prevent him from becoming it. For he becomes their Self (ātman).”
This identity was already recognised in the Çatapatha Brāhmaṇa (X. vi. 3): “Even as the smallest granule of millet, so is this golden Purusha in the heart.... That self of the spirit is my self: on passing from hence I shall obtain that Self.”
We find everywhere in these treatises a restless striving to grasp the true nature of the pantheistic Self, now through one metaphor, now through another. Thus (Bṛih. Up. II. iv.) the wise Yājnavalkya, about to renounce the world and retire to the forest, replies to the question of his wife, Maitreyī, with the words: “As a lump of salt thrown into the water would dissolve and could not be taken out again, while the water, wherever tasted, would be salt, so is this great being endless, unlimited, simply compacted of cognition. Arising out of these elements, it disappears again in them. After death there is no consciousness;” for, as he further explains, when the duality on which consciousness is based disappears, consciousness must necessarily cease.
In another passage of the same Upanishad (II. i. 20) we read: “Just as the spider goes out of itself by means of its thread, as tiny sparks leap out of the fire, so from the Ātman issue all vital airs, all worlds, all gods, all beings.”
Here, again, is a stanza from the Muṇḍaka (III. ii. 8):—
As rivers flow and disappear at last
In ocean’s waters, name and form renouncing,
So, too, the sage, released from name and form,
Is merged in the divine and highest spirit.
In a passage of the Bṛihadāraṇyaka (III. vii.) Yājnavalkya describes the Ātman as the “inner guide” (antaryāmin): “Who is in all beings, different from all beings, who guides all beings within, that is thy Self, the inward guide, immortal.”