Sanskrit dialogues and teachings that form the final layer of the Vedas, conventionally dated to roughly 800–200 BCE, where Indian thought turned from fire sacrifice and the gods toward a single radical question: what is the self, and how is it related to the ultimate reality behind everything? Their most famous passages identify atman, the innermost self, with Brahman, the ground of all existence — tat tvam asi, that thou art. Later Vedanta schools read this equation in sharply different ways. As the philosophical foundation of Hinduism, the Upanishads shaped every major tradition that followed.
A young man named Nachiketas is sent to Death itself and refuses to leave until Death answers one question: what happens to a person when they die? In another teaching, a father dissolves salt in water and asks his son to taste it everywhere in the cup, then explains that the unseen reality filling everything is, in the same way, the boy's own deepest self. These are scenes from the Upanishads, the Sanskrit texts where Indian thought turned away from sacrificing to the gods and toward a stranger question. The boldest answer in their most famous passages is a single equation, tat tvam asi, "that thou art": the infinite reality behind the whole universe is identical with the self you already are.