The Śatapatha Brāhmaṇa, a vast priestly manual of the Vedic sacrifice, cites Yājñavalkya as an authority on how the rites should be performed. One ruling shows his manner. The text forbids the sacrificer to eat the flesh of a cow or an ox, and then records Yājñavalkya's reply: he eats it himself, as long as it is tender. The closing section of that same Brāhmaṇa is the Bṛhadāraṇyaka Upaniṣad, and there the ritual expert with the sharp tongue asks a different question. He no longer asks how the sacrifice works, but who the person performing it is, and his answers are among the oldest philosophy that survives in India.
The texts cannot say whether a single man stood behind the speaker. Scholars place the Upanishad around the seventh or sixth century BCE, and the years on this page are that date, not a lifetime. Steven Lindquist, a scholar of religion who followed the figure through more than a thousand years of literature, treats Yājñavalkya as a character the texts kept building and does not try to recover the man. Later tradition makes him the founder of the White Yajurveda, one of the Veda's schools of ritual, and tells that his first teacher, Vaiśampāyana, made him give back everything he had learned. A law book, the Yājñavalkya Smṛti, also carries his name and has him say that he received the forest teaching from the sun. Patrick Olivelle, its modern editor and translator, dates it to the fourth or fifth century CE, the work of an author whose name is lost.
In the Upanishad he wins his standing in a contest. King Janaka of Videha offers a thousand cows, with gold tied to their horns, to the most learned of the brahmins at his sacrifice. No one dares to claim them, and Yājñavalkya tells his pupil to drive them home before he has answered a single question. The challengers follow one by one. Gārgī, a learned woman, presses him twice, and he warns her not to ask beyond what may be asked. When Ārtabhāga, another challenger, asks what remains of a person after death, Yājñavalkya takes him by the hand and refuses to answer in public. The two go aside, and the text reports only that they spoke of action, karma: a person becomes good by good action and bad by bad.
What he teaches Janaka afterwards turns attention from the world to the one who sees it. When the sun and moon have set, the fire has gone out and speech is silent, a person still has a light, and that light is the self. The self sees but cannot be seen, and it can be described only by negation, neti neti, "not this, not this". In dreamless sleep, he tells the king, a person becomes like an ocean, a single seer without a second, advaita; Michael Witzel, an Indologist at Harvard, credits Yājñavalkya with coining that term. As a person acts, so he becomes, and as he desires, so he acts.
The last dialogue takes place at home. About to leave his household and wander, Yājñavalkya offers to divide his property between his two wives, Maitreyī and Kātyāyanī. Maitreyī asks whether the whole earth full of wealth would make her immortal. He answers that it would not, and that a husband, a wife or a son is dear for the sake of the self. Lindquist counts leaving household life among the things the literature says he did first, and the ideal of renunciation attributed to him became important to the Hindu orders of wandering ascetics.
Advaita Vedānta, the school that holds the self and Brahman to be one, later read its own doctrine in these dialogues; Śaṅkara, its founding teacher, wrote a commentary on the whole Upanishad. The dialogues themselves are centuries older than any school of Vedānta.