यच्छेद्वाङ्मनसी प्राज्ञस्तद्यच्छेज्ज्ञान आत्मनि ।
ज्ञानमात्मनि महति नियच्छेत्तद्यच्छेच्छान्त आत्मनि ॥ १३ ॥

yacchedvāṅmanasī prājñastadyacchejjñāna ātmani |
jñānamātmani mahati niyacchettadyacchecchānta ātmani || 13 ||

13. Let the intelligent man sink speech into mind, sink that into intelligence and intelligence into the great atman and sink that into the peaceful atman.

Shankara’s Commentary:

The text states the means of attaining that Yatchet, draw into. Prajnah, one having discernment. What? Vaky, i.e., speech. The word ‘vak,’ i.e, speech, is illustratively used to denote all the senses. Where? In the mind. The lengthening of the vowel in manasi is a Vedic license; and that mind, let him sink into gnana, i.e., intellect bright by nature; ‘intellect’ is called atman here; for, it pervades the mind and the other senses; therefore, it is their ‘Pratyagatman,’ i.e., internal principle; let him sink the intellect into the atman; ‘great,’ i.e., first born Hiranyagarbha. The meaning is, let him make his intelligence as clear in its nature as the first born; let him sink that great atman also into the peaceful atman, i.e., into the primary atman whose nature does not admit of any conditions, which is unmodified, which is within all and which is the witness of all the modifications of the intellect.