Chandogya Upanishad 6.8

uddālako hāruṇiḥ śvetaketuṃ putram uvāca svapnāntaṃ me somya vijānīhīti yatraitat puruṣaḥ svapiti nāma satā somya tadā sampanno bhavati svam apīto bhavati tasmād enaṃ svapitīty ācakṣate svaṃ hy apīto bhavati

Uddâlaka Âruni said to his son Svetaketu: 'Learn from me the true nature of sleep (svapna). When a man sleeps here, then, my dear son, he becomes united with the True, he is gone to his own (Self). Therefore they say, svapiti, he sleeps, because he is gone (apîta) to his own (sva).

sa yathā śakuniḥ sūtreṇa prabaddho diśaṃ diśaṃ patitvānyatrāyatanam alabdhvā bandhanam evopaśrayate evam eva khalu somya tan mano diśaṃ diśaṃ patitvānyatrāyatanam alabdhvā prāṇam evopaśrayate prāṇabandhanaṃ hi somya mana iti

'As a bird when tied by a string flies first in every direction, and finding no rest anywhere, settles down at last on the very place where it is fastened, exactly in the same manner, my son, that mind (the gîva, or living Self in the mind, see VI, 3, 2), after flying in every direction, and finding no rest anywhere, settles down on breath; for indeed, my son, mind is fastened to breath.

aśanāpipāse me somya vijānīhīti yatraitat puruṣo 'śiśiṣati nāmāpa eva tad aśitaṃ nayante tad yathā gonāyo 'śvanāyaḥ puruṣanāya ity evaṃ tad apa ācakṣate 'śanāyeti tatraitacchuṅgam utpatitaṃ somya vijānīhi nedam amūlaṃ bhaviṣyatīti

'Learn from me, my son, what are hunger and thirst. When a man is thus said to be hungry, water is carrying away (digests) what has been eaten by him. Therefore as they speak of a cow-leader (go-nâya), a horse-leader (asva-nâya), a man-leader (purusha-nâya), so they call water (which digests food and causes hunger) food-leader (asa-nâya). Thus (by food digested &c), my son, know this offshoot (the body) to be brought forth, for this (body) could not be without a root (cause).

tasya kva mūlaṃ syād anyatrānnāt evam eva khalu somyānnena śuṅgenāpo mūlam anviccha adbhiḥ somya śuṅgena tejo mūlam anviccha tejasā somya śuṅgena sanmūlam anviccha sanmūlāḥ somyemāḥ sarvāḥ prajāḥ sadāyatanāḥ satpratiṣṭhāḥ

And where could its root be except in food (earth)? And in the same manner, my son, as food (earth) too is an offshoot, seek after its root, viz. water. And as water too is an offshoot, seek after its root, viz. fire. And as fire too is an offshoot, seek after its root, viz. the True. Yes, all these creatures, my son, have their root in the True, they dwell in the True, they rest in the True.

atha yatraitat puruṣaḥ pipāsati nāma teja eva tat pītaṃ nayate tad yathā gonāyo 'śvanāyaḥ puruṣanāya ity evaṃ tat teja ācaṣṭa udanyeti tatraitad eva śuṅgam utpatitaṃ somya vijānīhi nedam amūlaṃ bhaviṣyatīti

'When a man is thus said to be thirsty, fire carries away what has been drunk by him. Therefore as they speak of a cow-leader (go-nâya), of a horse-leader (asva-nâya), of a man-leader (purusha-nâya), so they call fire udanyâ, thirst, i.e. water-leader. Thus (by water digested &c), my son, know this offshoot (the body) to be brought forth: this (body) could not be without a root (cause).

tasya kva mūlaṃ syād anyatrādbhyaḥ adbhiḥ somya śuṅgena tejo mūlam anviccha tejasā somya śuṅgena sanmūlam anviccha sanmūlāḥ somyemāḥ sarvāḥ prajāḥ sadāyatanāḥ satpratiṣṭhāḥ yathā nu khalu somyemās tisro devatāḥ puruṣaṃ prāpya trivṛt trivṛd ekaikā bhavati tad uktaṃ purastād eva bhavati asya somya puruṣasya prayato vāṅ manasi saṃpadyate manaḥ prāṇe prāṇas tejasi tejaḥ parasyāṃ devatāyām

'And where could its root be except in water? As water is an offshoot, seek after its root, viz. fire. As fire is an offshoot, seek after its root, viz. the True. Yes, all these creatures, O son, have their root in the True, they dwell in the True, they rest in the True. 'And how these three beings (devatâ), fire, water, earth, O son, when they reach man, become each of them tripartite, has been said before (VI, 4, 7). When a man departs from hence, his speech is merged in his mind, his mind in his breath, his breath in heat (fire), heat in the Highest Being.

sa ya eṣo 'ṇimaitad ātmyam idaṃ sarvam tat satyam sa ātmā tat tvam asi śvetaketo iti bhūya eva mā bhagavān vijñāpayatv iti tathā somyeti hovāca

'Now that which is that subtile essence (the root of all), in it all that exists has its self. It is the True. It is the Self, and thou, O Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.