The earliest Veda, the Rigveda, presents a metaphysics that in our time we will understand as naive realism or commonsense realism. But the metaphysics of the Upaniṣads is much more diversified—along with naive realism, idealism, and skepticism, the Upaniṣads have verses that make references to concepts similar to the Platonic Forms and the Kantian noumena. For instance, there are verses in the Bṛhadāraṇyaka Upaniṣad which reject any attempt to investigate the noumenal universe through characteristics of the phenomenal universe—it theorizes in three verses (verse 2. 3. 6, verse 3.9. 26, and verse 4.2.4) that the noumenal nature of the universe cannot be defined by any characteristic of the phenomenal universe and that the noumena can be recognized only in terms of negative definition: “Neti, neti” (Not thus! not so!).