Mandukya Upanishad, verse 11

सुषुप्तस्थानः प्राज्ञो मकारस्तृतीया मात्रा मितेरपीतेर्वा मिनोति ह वा इदं सर्वमपीतिश्च भवति य एवं वेद ॥ ११ ॥

suṣuptasthānaḥ prājño makārastṛtīyā mātrā miterapītervā minoti ha vā idaṃ sarvamapītiśca bhavati ya evaṃ veda || 11 ||

11. Prājña whose sphere is deep sleep is M (म) the third part (letter) of Aum, because it is both the measure and that wherein all become one. One who knows this (identity of Prājña and M) is able to measure all (realise the real nature of the world) and also comprehends all within himself.

Shankara Bhashya (commentary)

One who is Prājña associated with deep sleep is M (म) the third sound (letter) of Aum. What is the common feature? It is thus explained. Here this is the common feature: The word Miti in the text means “measure”. As barley is measured by Prastha (a kind of measure), so also Viśva and Taijasa are, as it were, measured¹ by Prājña during their evolution (utpātti) and involution (praḷaya) by their appearance from and disappearance into Prājña (deep sleep). Similarly² after once finishing the utterance of Aum when it is re-uttered, the sounds (letters) A and U, as it were, merge into and emerge from M. Another common feature is described by the word “Apiteh” which means “becoming one”. When the word Aum is uttered the sounds (letters) A and U become³ one, as it were, in the last sound (letter) M. Similarly, Viśva and Taijasa become one (merge themselves) in Prājña in deep sleep. Therefore Prājña and the sound M are identical on account of this common basis that underlies them both. Now is described the merit of this knowledge. (One who knows this identity) comprehends all this, i.e., the real⁴ nature of the universe. Further he realises himself as the Ātman, the cause of the universe, i.e., Īśvara. The enumeration of these secondary⁵ merits is for the purpose of extolling the principal means (of knowledge).

Anandagiri Tika (glossary)

¹ Measured— Both the waking and dream states appear (during their evolution) from and disappear (at the time of their involution) into deep sleep. Therefore Prājña is, as it were, the container in which Viśva and Taijasa are contained. The nature of Viśva and Taijasa (non-apprehension of Reality) is known from the nature of Prājña—because it is the cause of the two other states. Therefore Prājña is here described as the measure of the two other states.

² Similarly—When the word ‘AUM’ is uttered quickly several times, the sound actually heard is Maum and not Aum, in which case it may be said that the sounds A and U emerge out of and merge into M.

³ Become one—i.e.., merge themselves.

⁴ Real Nature—That is, the universe experienced in the dream and waking states is of the same stuff as the Prājña.

⁵ Secondary merits—The enumeration of these secondary merits is for the satisfaction of those that still move in the causal plane.