All this is truly Brahman. This Ātman is Brahman. This very Ātman has four feet (aspects).

A fundamental metaphysical statement that establishes the structure of the Upaniṣad:

Sarvaṃ hy etad brahma — All this is truly Brahman. The entire universe is Brahman, not separate from It.

Ayam ātmā brahma — This Ātman is Brahman. Identity between the individual Self and the Absolute. It is one of the mahāvākyas (great sentences).

So ‘yam ātmā catuṣpāt — This very Ātman has four aspects (pāda = foot, part, aspect).

The four “feet” or aspects that will be developed are:

1. Vaiśvānara / Jāgrat — Waking state
2. Taijasa / Svapna — Dreaming state
3. Prājña / Suṣupti — Deep sleep state
4. Turīya — The fourth, beyond the three

The quaternary structure is the framework of the entire Upaniṣad: three ordinary states plus a fourth that transcends and grounds them.

The word catuṣpāt (four-footed) also evokes the image of Brahman as a cosmic being, only one quarter visible in the manifested world.