Nisargadatta MaharajThe sense of being, of 'I am' is the first to emerge. Ask yourself whence it comes, or just watch it quietly. When the mind stays in the 'I am' without moving, you enter a state that cannot be verbalized but can be experienced. All you need to do is try and try again.

Nisargadatta Maharaj

Aitareya Upaniṣad Reference Materials:

Video/Audio Lectures

Swami Paramarthananda
14 classes (~55 mins each)

Swami Nikhilananda
7 classes (~75 mins each)

Aitareya Upanishad - Introduction, Ch 1, Sec 1 - Shloka 2 to 4