Why are Vedas often referred to as "three Vedas" instead of four? Why is Atharva Veda excluded from this list?

Question

Āpastamba 1.1.28-29: If the proper time for initiation has passed, he shall observe, for the space of two months, the duties of a Student, as observed by those who are studying the three Vedas; after that he may be initiated; and after that he may be instructed.

Manu 3.1 (Period of Studentship): Duties relating to the Three Vedas should be observed under the Preceptor for thirty-six years, or for half that period, or for a quarter, or precisely till they have been got up.—(3.1)

Vālmīki Rāmāyaṇa 6-105-13: He [Surya, the sun-god] is the lord of the sky, the disperser of darkness, the master of the three Vedas (viz. Rik, Sama and Yajur), the sender of thick rain, the friend of water and the on who courses swiftly in the path of the sky.

Chāndogya Upaniṣad 2.21.1: The three vidyās [the Ṛk, the Yajuḥ, and the Sāma] are together the hiṃkāra; these three worlds [the earth, the space between the earth and heaven, and heaven] are together the prastāva; fire, air, and the sun are together the udgītha; the stars, the birds, and the rays are together the pratihāra; serpents, gandharvas, and the ancestors are together the nidhana. This Sāma resides in everything.

(Word-for-word explanation):

Trayī vidyā hiṃkāraḥ, the three Vedas [the Ṛk, the Yajuḥ, and the Sāma] are the hiṃkāra; ...

Is the reference usually to the three types of Vedic mantras that can be present in any of the four Vedas or to the three Vedas, namely, Ṛgveda, Yajurveda and Sāmaveda? If it's the latter, why is Atharva Veda excluded from the list?

Answer 1

I have come across two different answers to this interesting question.

The first answer according to some western Indologists is that Atharva Veda was recognized as a Veda very late. This explains the mention of only 3 Vedas in the various texts mentioned above.

The second answer is that the Atharva Veda mantras are not used in Yajnas and so those who want to do Yajnas do not have to read Atharva Veda.

The Atharvan is very much in the same position as we shall find the Yagus texts: the three Vedas are mentioned, often in connection with other more specific forms and designations of prayer and sacerdotal acts, but the Atharvan is omitted. The impression left .. is by no means that of conscious neglect or contempt, but rather of esoteric restriction to the sphere of the great Vedic ritual (srauta). Thus it augurs no contempt or neglect of the Atharvan, if in a charm constructed for the purpose of obtaining a knowledge of the Vedas, A.V. VII, 54 (Kaus. 42, 9), only rik, saman, yaguh, veda, and oblation (havih) are mentioned: the person who desires Vedic learning is not in training for Atharvan priesthood, and therefore does not care to include this specialistic learning.

Introduction to Hymns of the Atharva-Veda translated by Maurice Bloomfield

Answer 2

Why is Atharva Veda excluded from this list (trayī vidyā or trayo vedā)?

The answer really depends on the philosophical school of thought one subscribes to.

As Jayanta Bhaṭṭa of Nyāya school explains in his Nyāya-Mañjarī, the Mīmāṁsakas did not believe that Atharva Veda was either authentic or apauruṣeyā and it is for this reason a lot of Śruti and Smṛti texts exclude it from their list of Vedas:

An objection to the validity of the Atharva Veda

1. the Ṛg Veda, the Yajur Vedas and the Sāma Veda teach us interrelated subject-matters. Hence we infer that these three Vedas have been composed by the same author. They are the source of valid knowledge as it is an inference that they owe their existence to one and the same author. But the Atharva Veda is entirely different from the above three Vedas since it has no concern with the religious rites mentioned in them. For this very reason it cannot be a source book of the religious rites. The Ṛg Veda, the Yajur Veda and the Sāma Veda enjoin the biggest religious acts such as the Soma-sacrifices Jyotiṣṭoma etc. Instructions regarding these sacrifices have been given in the different recensions of the Vedas. The Brahmins who are well-versed in these three Vedas can only take part in the observance of these rites. Therefore, the Vedas which enjoin such religious acts are only valid. The Atharva Veda is not so.

   ...
   

2. Śruti and Smṛti bear evidence to the above view.
   
   

   1. The passages quoted from the Taittirīya Brāhmaṇa (III. 12. 9. 1), the Śatapatha Brāhmaṇa (XI. 5. 8. 1-3) and the Nārāyaṇa Upaniṣad corroborate this view. The relevant portion in the passages is that they refer only to the three Vedas.
      
      
   2. The Saṃhitā of Manu strengthens this view. The Saṃhitā says that one should observe the vow of celibacy for a period of twelve years in order to study the different Vedas. A Vedic student should reside in the house of his preceptor for 36 years, and read the three Vedas viz., the Ṛg Veda, the Yajur Veda and the Sāma Veda. He should thus observe the vow of religious study. Manu has also stated in his section on funeral ceremony that one who performs funeral ceremony should earnestly feed Brāhmaṇas who have made a complete study of the Ṛg Veda, the Yajur Veda and the Sāma Veda together with the corresponding Brāhmaṇas and the different recensions of the Vedas. Manu makes mention only of those Brāhmaṇas who have made a complete study of the three Vedas as entitled to the funeral feast. But he does not make mention of the Brāhmaṇas who have studied the Atharva Veda. Nay, in some cases, prohibition is noticed. A Brahmin who is well-versed in the Atharva Veda should not be invited to do this and that act.

Jayanta Bhaṭṭa then proceeds to refute the Mīmāṁsaka arguments:

A reply to the said objections

...

The Śatapatha Brāhmaṇa commences a topic with the remark that the Ṛg Veda is the vital breath of Brahman and completes it with the statement that a Brāhmaṇa well up in the Atharva Veda is equal to Brahman.

...

The Taittirīya Upaniṣad says that besides the sheath known as Prāṇamaya there is another inner sheath called Manomaya. It makes a number of statements of this sheath. In this connection it remarks that the Yajur Veda constitutes its head, the Ṛg Veda its right wing, the Sāma Veda its left wing, the Brahman its soul and the Atharva Veda its tail.

The Taittirīya says

To face the east is better for the recitation of the Ṛg Veda, to face the south is better for the recitation of the Yajur Veda, to face the south is better for the recitation of the Sāma Veda and to face the west is better for the recitation of the Atharva Veda.

In the Śatapatha Brāhmaṇa the following statements about the holy study of the Vedas are found. It commences with the statement that the mantras of the Ṛg Veda are the five mid-day oblations of milk unto gods. In this context it has been stated that the mantras of the Atharva Veda are the oblations of omentum to be offered to gods. He who studies the Veda everyday with this knowledge pleases gods with the offer of the oblation of omentum. Gods, being satisfied, incur his pleasure. The mantras of the Taittirīya Saṃhitā throw some light on the meaning of the above statement. O Fire! a person proficient in the Atharva Veda, has kindled you from Puṣkara. In the above mantras we find the word 'Atharva'. It is not the name of a particular sage. Such an interpretation does not stand to reasons since similar words, found in the other Vedas, may shake our confidence in the truth of the Vedas. We have cited passages from the Saṃhitas, Brāhmaṇas, Upaniṣads etc. in order to prove the authenticity of the Atharva Veda.

...

There is no such distinct book as goes by the name "trayī" (triplet). Though the collection of the Ṛg Veda, the Yajur Veda and the Sāma Veda is loosely called trayī collection yet it has space enough to include other mantras...The three Vedas jointly advise the duties of a Brahman priest. But none of them is the triple Vedas. The Atharva Veda alone is the triplet. As the Ṛg Veda, the Yajur Veda and the Sāma Veda are included in the Atharva Veda so the Atharva Veda having helped to discharge the duties of a Brahman priest, the three Vedas jointly perform them.

...

Some invite our attention to the statements "No body should have connection with a Brahman who has exclusively read the Atharva Veda". We have no regard for it since it is a statement of the Kalpa Sūtra and moreover is contrary to the spirit of the Vedas. If the above statement would have been a Vedic sentence then it had been interpreted in the light of the proper context. Most probably it has bearing only upon a particular Vedic rite. When that particular Vedic rite will be performed no scholar of the Atharva Veda should be appointed as a priest. It is an unconditional general statement then a conflict between the two Vedic sentences becomes inevitable. These two contradictory statements must be reconciled. We have cited Vedic sentences which accept the Atharva Veda as one of the Vedas. The prohibitive sentence suggests that the Atharva Veda falls outside the scope of the Vedas. The Vedas cannot blow hot and cold in the same breath. Therefore, the prohibitive sentence must have a restricted meaning. A scholar of the Atharva Veda should not be appointed as a priest in connection with an act where his sacrifices are not required.

Answer 3

Vyasa when he categorized the vedas is the one who split the then existing veda into their modern classifications - Rg, Yajur, Sama, and...the Atharva. Most of the Atharva consists of mantras for various spells and incantations, but, nevertheless, it is not entirely that. All modern Hindus (not including Shavites, and Yoga schools) are followers of Vyasa, the Uttara Mimaamsa - no matter whether followers of Sankara (Advaita - Monism), Ramanuja (Visistadvaita - qualified Monism), or Madhva (Dvaita - Dualism), and a few others. All are followers of the three Prasthanas, the Upanishads, the Brahma Sutras, and the Gita. Sankara, Ramanuja, Madhva, and others, all have written Bhashyas (commentaries) on Vyasa's Brahma Sutras. The Mundaka Upanishad I.i.4-5 says Swami Gambhirananda translator):

4. To him he said, '"There are two kinds of knowledge to be acquired--the higher and the lower"; this is what, as tradition runs, the knowers of the import of the Vedas say.'

5. Of these, the lower comprises the Rg-Veda, Yajur-Veda, Sama-Veda, Atharva-Veda, the science of pronunciation etc., the code of rituals, grammar, etymology, metre, and astrology. Then there is the higher (knowledge) by which is attained the Imperishable.

An objection can be raised that the Mundaka Upanishad (as is the Prasna Upanishad) is part of the Atharva Veda, so the Mundaka Upanishad verses above are self-referential. But, Vyasa has included over 100 references to the Mundaka Upanishad in his Brahma Sutras, as well as references to the Prasna Upanishad.

Remember that almost all (except for 8 verses if memory serves me right) of the Sama Veda is repetition of the Rg Veda. The Atharva Veda is not mentioned in the Chandogya Upanishad verse mentioned above as the for the completion of Dharma and Moksha, for the following of Rtam and Satyam - the Atharva Veda is not needed. You can also argue that the Chandogya verse is self-referential also, as the Chandogya Upanishad is part of the Sama Veda.

The final argument is except for the Chandogya verse mentioned, the other references are smriti, not sruti. Since the Mundaka is sruti, sruti always takes precedence over smriti.