Yāska's Nirukta 13.8

13.8

``svaryanto nāpekṣanta ā dyām rohanti rodasī/
yajñam ye viśvatodhāram suvidvāṃso vitenire/''.
svar gacchanta ījānā vā na īkṣante/
te'amum eva lokam gatavantam īkṣantam iti/
ā dyām rohanti rodasī/
yajñam ye viśvatodhāram sarvatodhāram suvidvāṃso vitenira iti/
atha eṣā vācaḥ pravalhitā iva/
``catvāri vākparimitā padāni tāni vidur brāhmaṇā ye manīsinah/
guhā trīni nihitā neṅgayanti turīyam vāco manuṣyā vadanti/''.(13,9)
catvāri vācaḥ parimitāni padāni/
tāni vidur brāhmaṇā ye medhāvinah/
guhāyām trīni nihitāni na artham vedayante/
guhā gūhates turīyam tvarateh/
katamāni tāni catvāri padāni/
om kāro mahā vyāhṛti1p ca ity ārṣam/
nāma ākhyāta1d ca upasarga nipātāś ca iti vaiyākaraṇāh/
mantraḥ kalpo brāhmaṇaṃś caturthī vyāvahārikī iti yājñikāh/
rco yajus1p sāmāni caturthī vyāvahārikī iti nairuktāh/(13,9)
sarpānām vāc vayasām kṣudrasya sarīsṛpasya caturthī vyāvahārikī ity eke/
paśusu tūnaveṣu mṛgeṣv ātmani ca ity ātma pravādāh/
atha api brāhmaṇam bhavati/

"Going to heaven, they look not around; they ascend heaven and earth,
who, knowing well, have extended the sacrifice that holds all."
Going to heaven, having sacrificed, they do not look back;
they look upon him who has gone to that very world.
They ascend heaven and earth,
who, knowing well, have extended the sacrifice that holds all, having supports on all sides.
Now this [verse] of Speech is like an enigma:
"Speech is measured in four quarters; these the Brāhmans who are wise know.
Three kept in secret do not stir; men speak the fourth [quarter] of speech."
Speech has four measured quarters.
These the Brāhmans who are wise know.
Three kept in secret do not reveal their meaning.
*Guhā* [secret/cave] is from the root *guh* [to hide]; *turīya* [fourth] is from the root *tvar* [to hasten].
Which are those four quarters?
"The syllable *oṃ* and the great utterances", according to the tradition of the ṛṣis.
"Noun, verb, preposition, and particle", say the grammarians.
"Mantra, ritual rule, Brāhmaṇa, and fourth the everyday [speech]", say the ritualists.
"The Ṛcs, the Yajuses, the Sāmans, and fourth the everyday [speech]", say the etymologists.
"The speech of serpents, of birds, of small creeping vermin, and fourth the everyday [speech]", say some.
"In cattle, in musical pipes, in wild beasts, and in oneself", say the teachers of the self.
Moreover, there is a Brāhmaṇa passage: