Some said that in the beginning there was nothing, and that what exists came out of what does not. The father who teaches in the sixth book of the Chāndogya Upaniṣad asks how being could come out of non-being, and answers that in the beginning there was only being, sat, one without a second. Being wished to become many and brought forth heat; heat brought forth water, water brought forth food, and everything that exists is some mixture of the three. The father is Uddālaka Āruṇi, and the whole book is his instruction to his son.
The Upanishad is anonymous, composed and handed down by memory, and Uddālaka survives only as a voice in it and in other Vedic texts. Those texts make him a brahmin of the Kuru and Pañcāla country and the teacher of many pupils, Yājñavalkya among them; in the Bṛhadāraṇyaka Upaniṣad that pupil defeats him in a contest of questions at the court of King Janaka. Patrick Olivelle, a modern translator of the Upanishads, dates the Chāndogya to the seventh or sixth century BCE, give or take a century. The years on this page are that age, not a lifetime.
The book opens on a failure of learning. Uddālaka sends his son Śvetaketu away to study, since no one in their family is a brahmin by birth alone. The boy leaves at twelve and comes back at twenty-four, having learned all the Vedas and proud of it. His father asks whether he ever asked for the teaching by which the unheard is heard and the unknown known. Śvetaketu has not, and supposes that his teachers did not know it. Uddālaka begins with clay. Know one lump and you know everything made of clay, because a pot differs from it only by a name, a word in speech; the clay is what is real.
Then he teaches by demonstration. He has Śvetaketu fast for fifteen days, until he cannot recite the Vedas, and then eat and recite them again, to show that the mind comes from food. He has him split the seed of a banyan fig until nothing is left to see, and leave salt in water overnight and taste it everywhere without finding it. The Indian philosopher Debiprasad Chattopadhyaya called Uddālaka the first natural scientist, and Hermann Jacobi, a German Indologist, compared him with Thales of Miletus, the first Greek philosopher. Nine times a lesson ends on the same sentence: that finest essence is the self of everything that exists, it is the real, and tat tvam asi, you are that, Śvetaketu.
Uddālaka gives his son images for why the essence goes unnoticed. The juices that bees gather from many trees become one honey and no longer know which tree each came from; rivers that run into the sea no longer know which river they were. A man led blindfold away from Gandhāra and left in a wilderness gets home only when someone loosens the bandage and points the way, and then by asking from village to village; so, the father says, a man who has a teacher comes to knowledge.
Tat tvam asi became one of the great sayings of Vedānta, the tradition of thought built on the Upanishads, and its teachers have divided over them ever since. Śaṅkara read them as saying that the self and Brahman, the ultimate reality, are one; Madhva read them as a denial. The book ends on an ordeal: a man accused of theft grasps a heated axe, and the one who told the truth is not burned. Śvetaketu, the text says, understood.