In one of the later Upanishads, the Çvetaçvatara (iv. 10), the notion, so prominent in the later Vedanta system, that the material world is an illusion (maya), is first met with. The world is here explained as an illusion produced by Brahma as a conjuror (mayin). 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 atman with the world Atman. It is most forcibly expressed in a frequently repeated sentence of the Chhandogya 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 Brihadaranyaka (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 (atman)."
This identity was already recognised in the Çatapatha Brahmana (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 (Brih. Up. II. iv.) the wise Yajnavalkya, about to renounce the world and retire to the forest, replies to the question of his wife, Maitreyi, 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 Atman issue all vital airs, all worlds, all gods, all beings."