The two earliest Upaniṣads are the Bṛhadāraṇyaka and the Chandogya (Candogya) Upaniṣads – around 600 BCE.
The Upaniṣads may be considered a kind of reaction against the earlier Vedas and their preoccupation with ritual protocol instead of larger philosophical questions.
In the Chandogya Upaniṣad, we find the famous Sanskrit statement “tat tvam asi” (“you are that”). Tat = that, tvam = you, asi = is.
This statement sums up the idea that our perishable individual self is to be equated with the eternal world around us.