BṚHADĀRAṆYAKA UPANISHAD · 1.5.3

त्रीण्य् आत्मने ऽकुरुतेति:
मनो वाचं प्राणं तान्य् आत्मने ऽकुरुत ।
अन्यत्रमना अभूवं नादर्शम् अन्यत्रमना अभूवं नाश्रौषम् इति मनसा ह्य् एव पश्यति मनसा शृणोति ।
कामः संकल्पो विचिकित्सा श्रद्धाश्रद्धा धृतिर् अधृतिर् ह्रीर् धीर् भीर् इत्य् एतत् सर्वं मन एव ।
तस्माद् अपि पृष्ठत उपस्पृष्टो मनसा विजानाति ।
यः कश्च शब्दो वाग् एव सा ।
एषा ह्य् अन्तं आयत्तैषा हि न ।
प्राणो ऽपानो व्यान उदानः समानो ऽन इत्य् एतत् सर्वं प्राण एव ।
एतन्मयो वा अयम् आत्मा ।
वाङ्मयो मनोमयः प्राणमयः ॥

trīṇy ātmane ’kuruteti:
mano vācaṃ prāṇaṃ tāny ātmane ’kuruta ।
anyatramanā abhūvaṃ nādarśam anyatramanā abhūvaṃ nāśrauṣam iti manasā hy eva paśyati manasā śṛṇoti ।
kāmaḥ saṃkalpo vicikitsā śraddhāśraddhā dhṛtir adhṛtir hrīr dhīr bhīr ity etat sarvaṃ mana eva ।
tasmād api pṛṣṭhata upaspṛṣṭo manasā vijānāti ।
yaḥ kaśca śabdo vāg eva sā ।
eṣā hy antaṃ āyattaiṣā hi na ।
prāṇo ’pāno vyāna udānaḥ samāno ’na ity etat sarvaṃ prāṇa eva ।
etanmayo vā ayam ātmā ।
vāṅmayo manomayaḥ prāṇamayaḥ ॥

When it is said, that 'he made three for himself,' that means that he made mind, speech, and breath for himself. As people say, 'My mind was elsewhere, I did not see; my mind was elsewhere, I did not hear,' it is clear that a man sees with his mind and hears with his mind. Desire, representation, doubt, faith, want of faith, memory, forgetfulness, shame, reflexion, fear, all this is mind. Therefore even if a man is touched on the back, he knows it through the mind. Whatever sound there is, that is speech. Speech indeed is intended for an end or object, it is nothing by itself. The up-breathing, the down-breathing, the back-breathing, the out-breathing, the on-breathing, all that is breathing is breath (prâna) only. Verily that Self consists of it; that Self consists of speech, mind, and breath.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)