who worships the one or the other of these, has not (correct) knowledge. . . . One should worship it as the Self. For in it all these (breath, etc.) become one."
In one of the later Upanishads, the Çvetāçvatara (iv. 10), the notion, so prominent in the later Vedānta system, that the material world is an illusion (māyā), is first met with. The world is here explained as an illusion produced by Brahma as a conjuror (māyin). This notion is, however, inherent even in the oldest Upanishads. It is virtually identical with the teaching of Plato that the things of experience are only the shadow of the real things, and with the teaching of Kant, that they are only phenomena of the thing in itself.
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
