The Brahma-conception as stated in Upaniṣad, has been hailed in the highest esteem in Mahābhārata and Purāṇa. The narrator of Mahābhārata itself, has established the greatness of the text, by calling it a narrative of elaboration of all that is in Veda and Upaniṣad–
brahman-vedarahasyañca yaccānyat sthāpitaṃ mayā
sāngopaniṣadāñcaiva vedānāṃ vistarakriyā.
To describe the level of the knowledge of sages and wise people, it is repeatedly said that ‘they are well-versed in Veda, Vedānga and Upaniṣad–
sāngopaniṣadān vedān caturākhyāna pañcamān
vedopaniṣadāṃ vettā ṛṣiḥ suragaṇārcitaḥ.

Since the theoretical knowledge of Upaniṣad speaks of nivṛtti from saṃsāra, since they show they path of self-investigation, the dharma of sannyāsa (reclusive life) or the fourth phase of Carturāśrama has been identified as the dharma of Upaniṣad.
caturthaścaupaniṣado dharmo sādhāraṇa smṛtaḥ
vānaprastha gṛhasthāśca tato’nyaḥ saṃpravartate
caturthopaniṣaddharmaḥ sādhāraṇa iti smṛtiḥ.
saṃsiddhaiḥ sādhyate nityaṃ …
yatidharmaḥ sanātanaḥ.
In the time of Mahābhārata, the grace of the knowledge of Upaniṣad was held so ideally valid, that the meaning of Upaniṣad became dharma, mantra or vīja. For instance, the dharma of a gṛhastha (householder) is to perfporm yāga-yājña with the wealth achieved legitimately, to serve the guests–
saiṣā gṛhasthopaniṣatpurāṇī.

Again, satya(truth) is the root of veda, conttrol over indriya(s)(dama) is the root of truth, dāna is the root of saṃyama (control), tapasyā is at the basis of dāna, renouncement is the root of tapasyā, and satisfaction or happiness lies at the root of renouncement. The term ‘Upaniṣat’ has been used to describe the worth of this ‘root’, which ultimately reveals the holistic meaning of Upaniṣad–
vedasyopaniṣat satyaṃ satyasyopaniṣaddamaḥ
damasyopaniṣaddānaṃ dānasyopaniṣattapaḥ
tapasyopaniṣattyāgastyāgasyopaniṣat sukham
sukhasyopaniṣat svargaḥ svargasyopaniṣacchamaḥ
In another context of Mahābhārata, ‘Upaniṣad’ is used in the sense of ‘root’, reinforcing the idea that Upaniṣad is the tatva of attaining mokṣa. Here it is said that truth is the root of Veda, dama or the power to control the indriya(s) is at the root of truth, and mokṣa or the desire for salvation is at the root of dama–
vedasyopaniṣat satyaṃ satyasyopaniṣaddamaḥ
damasyopaniṣanmokṣaḥ etat sarvānuśāśanam.
The earliest mention of the term Upaniṣad, along with the term niṣat, can be found in Khilasūkta of Śākala-Saṃhitā–
evā niṣaccāpaniṣacca viprā yuvāṃ rebhatau.
Scholars have said that this ṛk is difficult to understand, and therefore, they have taken help from Mahābhārata. There, in the praise of Kṛṣṇa uttered by Bhīṣma, it is said that in Veda-mantra(s), in the Brāhmaṇa-texts that reveal Vedic karma, and , and in the niṣat-texts which relate the elements of Vedic rituals, and the meaning of mantra(s),

yaṃ vākeṣvanuvākeṣu niṣatsūpaniṣatsu ca.
Here vāk is Vedic mantra-part, anuvāk is Brāhmaṇa, and niṣat is Āraṇyaka– as Nīlaṇṭha seeks to explain, and by ‘Upaniṣad’ he refers to Self-knowledge–
niṣatsu karmāṅgādyavavaddha devatājñādivākyeṣu upaniṣatsu kevalātmajñāpaka vākyeṣu.
In the original śloka of Mahābhārata, vāk-anuvāk is Veda-Brāhmaṇa and in its consequent order, Niṣat should be a definitional term for Āraṇyaka — what Nīlakaṇṭha has explained. So, this explanation is very much valid, and when Keith calls it absurd, it appears that he could not properly undersand the tradition of Indian philosophical thoughts.

Without having understood this, he has written, The epic invests an absurd Niṣad as a form of literature besides Upaniṣad.

The term niṣad is obviously a definitional usage of Āraṇyaka, because the process of asking about deep theories of knowledge being ‘niṣanna’ or seated in front of the Ācārya had its origin in the Āraṇyaka texts. Anirvan, a seer of modern times , citing the use of ‘niṣat’ from Ṛkveda, has said, niṣat is that profound concept, which makes one feel, after the utterance of mantra, that the god is niṣanna of upaviṣṭa in our mind (abhisvavā niṣadā gā etc.Ṛkmantra, 2.21.5. Niṣat means the influence of god, where there is divine influence internalised, is called Upaniṣt, as stated by Ṛgveda itself–
*raṇā vā ye niṣadi satte asya/
purā vividre sadu nūtanāsaḥ.
*arādhi hotā niṣadā yajīyān.

This internalised influence of God has been called dīkṣā in Śatapatha Brāhmaṇa. At the time of dīkṣā, the mantra uttered by the guru is internalised into the mind and body of the disciple–
yā vai dīkṣā sā niṣat

The sense of god’s internalisation in the term niṣat, and the identification of niṣat and dīkṣā have added much to the profound significance of ‘Upaniṣat’. In Ṛgveda, Upaniṣat is called the rasa of Yajurveda, in Śatapatha and Aitareya Brāhmaṇa, vākya is upaniṣat, and that upaniṣad is Vedaśira, the head of Veda–
tāmetām upaniṣadaṃ vedaśiro na yathā
kathañcana vadet. tadetat ṛcābhyuditam.

It is said that Upaniṣad is the head of Ṛgveda, the uttamāṅga or upper portion of Yajurveda, the śira of Sāmaveda and the muṇḍa of Atharvaveda

ṛcāṃ mūrdhānaṃ yajuṣāmuttamāṅgaṃ/
sāmnāṃ śiro’tharvaṇāṃ muṇḍamuṇḍam
nādhīte’dhite vedamāpustamajñam/
śiracchitvāsau kurute kavandham.

Two or three kinds of meaning of Upaniṣad, but the most acceptable meaning is given by the wisest commentator Śankarācārya. According to him, ‘sad’ means ‘to digest’, to destroy or to go. And the upasarga ‘ni’ means ‘certainly’ or ‘to the end’. The upasarga ‘ni’ means ‘near’. The vidyā knowledge which destroys avidyā, the cause of human life and death, and brings the soul, aspiring for salvation, to the Supreme Brahman. The chief meaning of Upaniṣad is Brahmavidyā (knowledge of Brahman), and the text, by learning which, one may attain the knowledge of Brahman, is also called Upaniṣad, in a secondary sense. The Upaniṣad text is called gauṇa or secondary, because not only by reading the text, one may achieve the knowledge of Brahman, but one has to sit (ni+sad)near (upa) the Ācārya, ask him about the knowledge. That is why these texts, relating the knowledge of Brahman, is called Upaniṣad.

Another name of Upaniṣad is ‘rahasya’. Rahaḥ means a secret place, rahasya means some secret knowledge. This profound knowledge is not to be disclosed anywhere, so the knowledge of Upaniṣad is ‘rahasya’. In Upaniṣad itself it is said that this knowledge is not to be given to anyone who is not ‘broad’ enough, to an unworthy disciple or unworthy son–
vedāste paramaṃ guhyaṃ purākalpe pracoditam
nāpraśastāya dātavyaṃ nāputrāyaśiṣyāya vā punaḥ.

According to the tradition of Śāstra(s), the Brāhmaṇa-texts were composed after the Veda(s), and after that the Āraṇyaka(s). The Upaniṣad texts were more or less contemporary with the major Upaniṣad(s). If the karmakāṇḍa (rituals related action) is related in Veda(s) and early texts, the jñānakāṇḍa is related in Upaniṣad(s).

It should be kept in mind that in the Brāhmaṇa texts, mainly karma, and secondarily jñāna have been discussed, but the portion which metaphorically and symbolically deals with both jñāna and karma, is called Āraṇyaka. The compsition and study of the Āraṇyaka texts were done inside the forests. Since these were too difficult and deep to understand, so they could be discussed here and there. Advices based on Āraṇyaka texts were done in secluded places within woodlands. The Upaniṣad(s) that have come down to us, through tradition, are mostly associated with Āraṇyaka texts. But considering their importance, the Upaniṣad(s) have been added to Veda. Sometimes they are also added to Brāhmaṇa texts.

For instance it can be said that Kena Upaniṣad is part of Jaiminya Brāhmaṇa of Sāmaveda. Vṛhadāraṇyaka Upaniṣad is included in Śatapatha Brāhmaṇa, but it is called Āraṇyaka Upaniṣd. The first part of Tāṇḍyamahābrāhmaṇa or the twenty fifth Brāhmaṇa of Sāmaveda is called Āraṇyaka, and Chāndogya Upaniṣad is included in this Āraṇyaka. Aitareya Upaniṣad is included in Aitareya Upaniṣad, and Taittirīya Upaniṣad is part of Taittirīya Āraṇyaka. The Upaniṣd(s) included in Āraṇyaka are called Āraṇyakopaniṣad, those included in Brāhmaṇa are Brāhmanopaniṣd and those included in the mantra-part of Veda are called Mantropaniṣad. There is only one mantropaniṣad which is not a part of Brāhmaṇa and Āraṇyaka– that is Īśopaniṣad. According to our conception, this can be categorised as follows–
Ṛgveda- Aitareya, Kauṣītaki, Sāmaveda– Chāndogya, Kena, Yajurveda– a) Kṛṣṇayajurveda, Taittirīya, Kaṭha, Śvetāśvatara, b) Śuklayajurveda, 8. Vṛhadāraṇyaka 9. Īśāvāsya or Īśa , Atharvaveda- 10. Praśna, 11.Muṇḍaka, 12. Māṇḍukya.

There are more Upaniṣad(s), about 200 in number, but they are mostly not authentic. Mainly these twlelve Upaniṣd(s) are described as authentic and ancient, because the great Śankarācārya has written commentaries on them.
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