Kaivalya Upaniṣad 21

Verse 15 tells us how everything including prāṇa and the five elements, all come from this atma alone. The meaning of prāṇa is beautifully explained in this talk along with, the two ways in which things are known- pramāṇa Siddha (objects that are revealed through a means of knowledge) & Swatah Siddha (self-revealing). Further, Ashvalayana's question on wanting to know this Brahman is answered in verse 16, thus revealing the mahāvākya which establishes a connection between the jiva (I) & Īśvara.