Mandukya Upanishad, verse 8


सोऽयमात्माध्यक्षरमोङ्कारोऽधिमात्रं पादा मात्रा मात्राश्च पादा अकार उकारो मकार इति ॥ ८ ॥

so'yamātmādhyakṣaramoṅkāro'dhimātraṃ pādā mātrā mātrāśca pādā akāra ukāro makāra iti || 8 ||

8. The same Ātman (which has been described above as having four quarters) is, again, Aum, from the point of view of the syllables (akṣaram). The Aum with parts is viewed from the standpoint of sounds (letters, mātrāḥ). The quarters are the letters (parts) and the letters are the quarters. The letters here are A, U and M.

Shankara Bhashya (commentary)

In the word Aum prominence is given to that which is indicated by several names. The word Aum which has been explained before as Ātman having four quarters is again the same Ātman described here from the standpoint of syllable where prominence is given to the name. What, again, is that syllable? It is thus replied: Aum. It is that word Aum which being divided into parts, is viewed from the standpoint of letters. How? Those which constitute the quarters of the Ātman are1 the letters of Aum. What are they? The letters are A, U and M. Tn the first Upaniṣad it is said, “Aum, the word, is all this.” The word Aum is the name (abhidhāna) which indicates everything (abhidheya) past, present, future and all that which is beyond even the conception of time. Thus Aum is the name for Brahman. The second Upaniṣad declares that Brahman is the Ātman. The Ātman with its four quarters has been explained in the following Upaniṣads. Therefore all these explanations are of Aum from the standpoint of Ātman where prominence is given to that which is indicated by names. Now the same Aum is explained from the standpoint of the word itself, that is the name which indicates Ātman or the Supreme Reality. The Highest Truth as explained above by the process of the refutation of the erroneous superimposition can be grasped only by the students of sharp or middling intelligence. But those ordinary students who cannot enter upon philosophical reflection regarding the Supreme Reality as given in the previous texts, are advised to concentrate on Aum as the symbol of the Ultimate Reality.