What is the validity of Authoritativeness of vedas?

Question

Pramana is defined as that which is the means of valid knowledge ( Prama)

In general three Pramanas are accepted by all vedantins.

1. Prtyaksha ( Perception)
2. Anumana ( Inference )
3. Sabda ( Verbal testimony)

In addition some school of thoughts accept more than three Pramanas . e.g. Mimamsakas as well as some advaitins accept Upamana (comparison), arthapatti ( Presumption) and anupalabdhi (non-apprehension)

Now, Vedas ( revealed scriptures) are verbal testimony and are considered apaurusheya (not authored by any human, divine revelation).

The question is what are arguments for the validity of Vedas and it's Authoritativeness?

Please quote from scriptures and commentaries of various acharyas to support the validity and authority of Vedas being apaurusheya?

Answer 1

Purva Mimamsa School in it's Purva Mimamsa Sutras gives very logical arguments to prove Eternity of Vedas. Actually these Arguments can also be used against Nastika school who hold Vedas are not eternal.

Quotes from Mahabharata, Ramayana, Puranas and other scripture to support Eternity of Vedas cannot be used as logic against Nastika school because they do not believe in these scripture too.

Hence we should go through Logical Analysis. Adhaya 1 Pada 1 of Mimamsa Sutras gives very logical reasons for eternity of Vedas. I have here prepared a concise summary with the Sutras itself. For those who are interested in reading detailed arguments one can read here.

First we Prove that Words are Eternal. For the logic that Words are eternal Purvapakshin (Objector) raises following objection

Objection 1:

One set says that it is an action; by reason of seeing it there. (Adhikarana 6 Sutra 6)

The objector says that word is not eternal as said in Sutra ; because it is an act and we see that it is produced by an effort.

Objection 2:

By reason of no stability. (Sutra 7)

The second objector says that there is no stability of the word ; as soon as it is pronounced, it vanishes.

Objection 3:

By reason of the word make . (Sutra 8)

The third objector says that the word { make is used in relation to l word . It is said, make a sound ; This fact also shows that sound is made and so it can not be eternal. in other beings ;by reason of its being simultaneous.

Objection 4:

By reason of its being heard simultaneously by other beings. (Sutra 9)

The fourth objection to the eternality of the sound is that it is heard simultaneously by all beings who stand at a distance from the source of the sound. This fact also goes, to establish the transitory nature of the sound, voice or word- original form modified form.

Objection 5:

And on account of the original and modified forms. (Sutra 10)

The changes take place, for instance expressions as 'Dadhyatri' we find original form we had 'i'  becomes modified into 'ya'. due to Grammar rules. As a rule anything liable to modification is not eternal.

Objection 6:

Its increase by reason of many persons pronouncing it. (Sutra 11)

There is another sixth argument against the eternality of the sound; when many persons together pronounce a word, it increases in sound. What increases and decreases is not eternal.

Now we proceed to Reply above objections:

Reply to Objection 1:

On the other hand there is a reasoning of equal force. (Sutra 12)

The first objector said it was an act and an effort ; that is to say it was pronounced. The author says you are quite right that it is a product of an effort i. e., pronunciation, but if the word did not exist before, it could not be pronounced. The very fact that it was pronounced shows that it existed before the pronunciation. It is by parity of reasoning that the contrary proposition is established.

Reply to Objection 2:

Disappearance after once coming into existence is by reason of the object not coming into contact. (Sutra 13)

The reply to the second objection is that the sound is the quality of ether. The ether is eternal. The sound was brought into existence, because it existed there and disappeared because the object is not obtain able by the sense though existing of the application.

Reply to Objection 3:

After the application. (Sutra 14)

The author replies the third objection which is about the verb make . It is also applicable to an effort to bring- into manifestation. When a word is pronounced it is audible, the audibility of the word is after the pronunciation. The word existed and the pronunciation only made it audible.

Reply to Objection 4:

The simultaeousness is like the sun. (Sutra 15)

The author replies the fourth objection. You say that sound is heard simultaneously by several persons standing there, so you say that there is not one sound. There you are wrong. There is one sun and spectators all see it; by the number of spectators the sun will not increase in number. So there is one sound and it is eternal.

Reply to Objection 5:

The change of letters is not a modification. (Sutra 16)

The changes of letters are no modifications, the word still exists and it is not modification because it is different letter.

Reply to Objection 6:

The increase is with reference to the increase of the tone. (Sutra 17)

The increase is of the tone or the noise; it is not the increase of the word.

Furthermore words are eternal because:

Reason 1:

On the other hand, it is eternal by the reason of its manifestation being for the sake of others. (Sutra 18)

The word is eternal, the reason is that it is for the sake of imparting information to others that it is pronounced and that the words come and go but the effect that they leave behind is permanent. The word cow is pronounced, the word as pronounced has disappeared but the knowledge of the cow that it has left on the mind of the hearer is still there even though the sound is not heard. everywhere.

Reason 2:

By reason of the simultaneity, it produces the same effect everywhere. (Sutra 19)

As soon as the word cow is pronounced, all the hearers understand that it means a particular animal of a particular shape. There are three terms, the first is the form which you have before your minds eye. You know the figure cow and you go examining each animal and when you find a particular animal corresponding to the figure that you have before your mind s eye, you at once recognize the individual cow.

Reason 3;

By reason of there being no number. (Sutra 20)

You pronounce the term cow ten times, it will convey the idea of one cow only. The word cow though pronounced several times conveys the idea of one individual cow.

Reason 4:

By reason of its being non-correlative. (Sutra 21)

There are correlative terms, such as husband and wife; father and son. You can not think one without the other. Sound may be produced or vanish but the word is absolute and not correlative; it exists independently and therefore eternal manifestation.

Reason 5:

And by reason of no manifestation of the collection of words. (Sutra 22)

The words are eternal, because the words in a collected form have no manifestation. The words collectively do not denote a class but only an individual word does so.

Reason 6:

It has no meaning when manifested, because the meaning does not depend upon it. (Sutra 24)

The objector says that I accept your proposition that the word and its sense are connected eternally but a number of words there is not so connected, because the sentence does not depend upon the words for its meaning; the meaning of the sentence is conventional. It of the constituent parts.

Reason 7:

The pronunciation of the constituent parts (words) is with the object of an action, the sense being dependant upon them. (Sutra 25)

The objector in the preceding sutra said that the words conveyed the sense but the words put into a sentence had only a conventional sense but no meaning of their own. We  reply you are wrong there ; the words are used for action and the sentence, being composed of the words which you admit to have a meaning, has necessarily a meaning.

Thus we have proved through logical reasoning that Words are eternal. Objector further raises other objection against Vedas. Such as:

Objection 1:

And the one party hold that the Vedas are modern being called after human names. (Sutra 27)

The objecter says that I accept that a word is eternally related to its sense and further admit that when put into a sentence it conveys a sense ; but where is the authority of the Vedas. They are of human origin being called after human names.

Objection 2:

And by seeing transitory things (in them). (Sutra 28)

The objector further develops his argument and says that the names of the human beings are mentioned in the Vedas and so they are the works after the birth and death of those human beings. So they are human products.

Reply to Objection 1:

On the other hand the priority of the word is already spoken of. (Sutra 29)

We have already established the eternity of the word ; so every word whether human or divine is eternal. The objection is therefore groundless.

Reply to Objection 2:

The name is on account of the reason explaining them. (Sutra 30)

Your argument that the Vedas are of human origin because they are called after human beings is untenable ; they are called after human names, because those great sages were first to expound them and so the different branches of the Vedas were called after those sages in their honour.

Actually Purva Mimamsa Sutra goes very long reasoning after these sutras to explain each and every objection raised against Vedas. However I think the above arguments are sufficient to establish eternity of Vedas:

Thus the process of establishing Eternity of Vedas goes by following process:

1) First we establish eternity of Words.

2) We know that Vedas are words and sounds. So as we proved Eternity of words in point 1. Thus Vedas are eternal.

3) We explain human names, transistory things which may seem to make Veda time subjective.

4) Thus finally we establish Vedas are Eternal.

Answer 2

To be a follower of the Sanatana Dharma (Hindu) means that you accept the authority of the Vedas. If you do not accept the authority of the Vedas, then you are heterodox, not a follower of the Sanatana Dharma. The Vedas do not directly say that they are the authority; what they do say is that they are directly from God (Brahman) and that all knowledge comes through them. Past Vedantic commentators have made arguments in favor of Brahman against the other schools extensively, but on the Vedas themselves, the arguments are on their eternity and as coming from Brahman. As an example, see Chapter 2 of the Brahma Sutras (here where extensive arguments are made on the existence of Brahman against the other schools (Samkaya, Buddhist, Nyaya, etc.).

But first let me address your beginning comments on pramanas. The list of pramanas is partially valid (no reference is offered as to why you state that the three mentioned are agreed upon by all Vedantists, nor a reference as to the others not agreed upon by all). In fact there are arguments made by Vedantists against including inference as a separate pramana (see Vedanta Paribhasa pages 48-49, available here. In fact the paramanas included in the list are those of the Nyaya school (see Bhasa Pariccheda by Visvanatha-Nyaya-Pancanana, verse 52. Commenting on this verse he quotes Gautama’s Nyaya-Sutras I.i.3 – “Perception, inference, comparison, and verbal testimony are the means.”). Your list is thus of the Nyaya school (the fourth, comparison, Upamana, is not in your list), and not Vedantists - but it allows you to segue into what appears to be a valid question. Pramanas have no bearing on the question you have at hand. Pramanas are ways that the mind gains knowledge, lower knowledge – “knowledge that is limited by the mental states in the form of particular objects” (commentary of Swami Madhavananda in the Vedanta Paribhasa). Your statement that the Vedas are 'verbal testimony' ("Now, Vedas (revealed scriptures) are verbal testimony...") is incorrect, but your opening statements about pramanas makes it appear as a valid question. The juxtaposition of the word valid to knowledge and the word validity to Vedas is also misleading as it gives the reader the impression that there is a ‘valid knowledge’ that is applicable to the Vedas in the same sense as you use the term valid knowledge as a pramana.

The Vedas are not verbal testimony. The Vedas are the revealed word of God. The Vedas are Verbal Authority. The Mundaka Upanishad (I. i. 1-6) says Brahma imparts to his lineage the higher knowledge (knowledge of Brahman) and lower knowledge (inclusive of the Rig, Yajur, Sama, and Atharva Vedas - see verse 5). Higher knowledge refers to actual realization of Brahman and does not imply knowledge gained through our minds. Sankara says in his commentary on verse 5 that the Upanishads are not included in the lower knowledge as the knowledge imparted by the Upanishads is realization of Brahman. The Katha Upanishad (II.iii.15) says: “When all the knots of the heart are destroyed, even while a man is alive, then a mortal becomes immortal. This much alone is the instruction (of all the Upanishads).” Meaning that the sole intent of the Upanishads is to impart the higher knowledge of Brahman.

The Brahma Sutras 1.3.28-30 also addresses the eternity of the Vedas. In his commentary to verse 29, Sankara quotes Rig Veda 10.71.3 – “By means of their past good deeds (the priests) attained the capacity to understand the Vedas; (then) they found them dwelling in the Rishis.” And in verse 28, Sankara’s commentary quotes Manu Smriti 1.21 – “The several names, actions, and conditions of all things He shaped in the beginning from the words of the Vedas.”

Brahma Sutras 1.1.3-4 also addresses that Brahman can be known only through scripture and that the main purport of all Vedanta texts is Brahman. In his commentary on verse 3, Sankara says “…Brahman has no form etc. and so cannot be cognized by direct perception [your first pramana]. Again, in the absence of inseparable characteristics, as smoke is of fire. It cannot be established by inference [your second pramana] or analogy (Upamana) [the fourth pramana of the Nyaya school]. Therefore It can be known only through the scriptures…”

Answer 3

I'm not (including/doing) arguments for that but let quote some Shruti and Smriti verses and quotes that say Vedas are Apaurusheya or the words of Parameshwara.

• According to Manusmriti:

  श्रुतिस्तु वेदो विज्ञेय:

  It means Vedas are Shruti (that which is heard)

  ऋषयो मन्त्रद्रष्टार:

  Sages are seer (not creator) of Vedic hymns.

  And

  धर्मं जिज्ञासमानानां प्रमाणं परमं श्रुतिः

  To those who seek the knowledge of the sacred law the supreme authority is the revelation (Sruti)
  
  

• According to Rigveda: (Purusha Sukta)

  tasmādyajñātsarvahuta ṛcaḥ sāmāni jajñire|
  chandāṃsi jajñire tasmādyajustasmādajāyata|| 10.090.09

  From this sacrifice called "All embracing" The chants of Rig Veda were born, The chants of Sama Veda were born, And from that the well-known meters were born, And from that Yajur Veda was born.

  From verse 5.57.8:

  सत्यश्रुतः कवयः

  Hearers of truth
  
  

• Mundaka Upanishad: (1.2.1)

  तदेतत् सत्यं मन्त्रेषु कर्माणि कवयो यान्यपश्यंस्तानि त्रेतायां बहुधा सन्ततानि । तान्याचरथ नियतं सत्यकामा एष वः पन्थाः सुकृतस्य लोके ॥ १॥

  That truth is this, that what observances the seers comprehended from the three Vedas and were practised intensely during theTreta age, you should also practise with devotion. Oh! Seekers of Truth! That is your path which will lead you to the world of those of meritorious deeds. (1).

  Also read verse 2.1.4 of Mundaka which mentons Vedas a speech of Paramatma.

• According to Brihadaranyak Upanishad: (2.4.10)

  स यथाऽऽर्द्रैधाग्नेरभ्याहितात्पृथग्धूमा विनिश्चरन्त्य् स यथा आर्द्रैधाग्नेस् अभ्याहितस्य अभ्याहिताद् पृथक् धूमास् विनिश्चरन्ति एवं वा अरेऽस्य महतो भूतस्य निःश्वसितमेतद् यदृग्वेदो यजुर्वेदः सामवेदोऽथर्वाङ्गिरस इतिहासः पुराणं विद्या उपनिषदः श्लोकाः सूत्राण्यनुव्याख्यानानि व्याख्यानान्य् एव वै अरे अस्य महतस् भूतस्य निश्वसितम् निःश्वसितम् एतद् यद् ऋग्वेदस् यजुर्वेदस् सामवेदस् अथर्वाङ्गिरसस् इतिहासस् पुराणम् विद्यास् उपनिषदस् श्लोकास् सूत्राणि अनुव्याख्यानानि व्याख्याननि अस्यैवैतानि निःश्वसितानि ॥ १० ॥

  "As from a fire kindled with wet fuel various kinds of smoke issue forth, even so, my dear, the Rig—Veda, the Yajur—Veda, the Sama—Veda, the Atharvangirasa, history (itihasa), mythology (purana), the arts (vidya), the Upanishads, verses (slokas), aphorisms (sutras), elucidations (anuvyakhyanas) and explanations (vyakhyanas) are like the breath of this infinite Reality. From this Supreme Self are all these, indeed, breathed forth.

• From Mahabharata Shanti Parva (232):

  At the outset the Self-born caused those excellent Vedic sounds, that are embodiments of knowledge and that have neither beginning nor end to (spring up and) flow on (from preceptor to disciple). From those sounds have sprung all kinds of actions. The names of the Rishis, all things that have been created, the varieties of form seen in existent things, and the course of actions, have their origin in the Vedas. 3 Indeed, the Supreme Master of all beings, in the beginning, created all things from the words of the Vedas. Truly, the names of the Rishis, and all else that has been created, occur in the Vedas. Upon the expiration of his night (i.e., at the dawn of his day), the uncreate Brahman creates, from prototypes that existed before, all things which are, of course, well-made by Him.

• According to Svetasvatara Upanishad (6.18):

  यो ब्रह्माणं विदधाति पूर्वं यो वै वेदांश्च प्रहिणोति तस्मै ।
  तं ह देवं आत्मबुद्धिप्रकाशं मुमुक्षुर्वै शरणमहं प्रपद्ये ॥ १८॥

  Seeking Liberation, I take refuge in the Lord, the revealer of Self—Knowledge, who in the beginning created Brahma and delivered the Vedas to Him.

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• Niruktakara Yaskacharya have written in his Veda-Bhashya that "Vedas are eternal and Apaurusheya". ^{(source - http://bharatdiscovery.org)}

• आचार्य लौगाक्षि भास्कर ने दार्शनिक दृष्टि रखते हुए- अपौरुषेय वाक्य को वेद कहा है।^[4]
  अपौरुषेयं वाक्यं वेद:।
  Aacharya Laugakshi has said Vedas are Apaurusheya words. ^{(source - http://bharatdiscovery.org)}

• A well known commentator on Vedas, Sayanacharya has mentioned following quote in preface of his commentary on Taittariya Samhita:

  प्रत्यक्षेणानुमेत्या वा यस्तूपायो न विद्यते ।
  एनं विदन्ति वेदेन तस्माद् वेदस्य वेदता ॥

  Which means by That which can not be known either by direct perception or by inference, that Reality can be known only through the Vadas

• From The Complete Works of Swami Vivekananda/Volume 2/Jnana-Yoga/Realisation:

  All that is called knowledge is in the Vedas. Every word is sacred and eternal, eternal as the soul, without beginning and without end.

• From The Complete Works of Swami Vivekananda/Volume 1/Addresses at The Parliament of Religions/Paper on Hinduism:

  The Hindus have received their religion through revelation, the Vedas. They hold that the Vedas are without beginning and without end.

• From The Complete Works of Swami Vivekananda/Volume 6/Writings:

  In matters of religious duty the Vedas are the only capable authority.....The whole body of supersensuous truths, having no beginning or end, and called by the name of the Vedas, is ever-existent.....The person in whom this supersensuous power is manifested is called a Rishi, and the supersensuous truths which he realises by this power are called the Vedas....The authority of the Vedas extends to all ages, climes and persons; that is to say, their application is not confined to any particular place, time, and persons.

Answer 4

Veda is Apaurusheya. All other Pramanas (means of knowledge) with different Aastika schools of thought accept Veda as the authoritative text and with different means arrive at the meaning of Vedas. But as for the validity of Veda (as far as I know) they do not talk. Because if we raise the question on the authority of Veda itself, then we cannot take anything as Pramana. But since you asked, I will quote only one sloka I know which I hope should serve the purpose.

In Bhagavad Gita, the Lord says

“Shruti:smriti: mamaiva AgyA-yastAm ullanghya vartatE AgyAcchEdI mama drOhI mama bhaktOpi na Vaishnava:”

Sruti and Smruti are my commands. He who violates them is a traitor.

---

Edit: I searched for the source of the above sloka which is not Bhagavadgita. So I am editing the answer to explain that it is difficult to prove the authority of the Vedas.

Vedas are Apaurusheya meaning they are not created by or verbal testimonies of human being or any other being, but they are revealed to Rishis and Gods who were in penance. The Lord himself says in Gita as तस्माच्छास्त्रं प्रमाणं ते कार्याकार्यव्यवस्थितौ (BG-Ch16-Verse24) meaning śāstra (vedas) is the means of knowledge for you (Arjuna), in the determination of what is to be and not to be done. You can take this as one very valid argument because the lord himself declared that they are the authority for everything.

Jagadguru Shankaracharya of Sringeri HH Sri Bharati Teertha Mahaswami explains in his Anugraha Bhashanam explains as

When we mention the term “Shastras”, we only refer to the Vedas. The word “Shastram” is derived from the verse – “शास्ति इति शास्त्रम्”. Only the Vedas can assertively say what needs to be done. It is very difficult to question what is mentioned in the Vedas. The Dharma Shastras mention that Vedas are the supreme authority on Dharma in the verse – वेदोऽखिलो धर्ममूलम्. Dharma refers to something that brings us good and Vedas list down the things that bring us good and the things that we are not expected to do. We should adhere to what is mentioned in the Shastras. The philosophers of yore have all stated – यच्छब्द आह, तदस्माकं प्रमाणम् – “Shabda” here refers to the Vedas. What the Vedas state is the authority for us.

There is a story in Tripura Rahasyam regarding the faith in the Shastras which goes like this

Once there lived a saint, by name Kausika, on the Sahya Hill near the banks of the Godavari. He was serene, pure, pious, having knowledge of the Supreme Truth. Several disciples attended on him. Once when the master had gone out, the disciples started to discuss philosophy, according to their own lights. There appeared on the scene a Brahmin of great intellect and wide learning, Soonga by name, who successfully refuted all their arguments by his skill in logic. He was a man without faith and without conviction, but an able debater. When they said that the truth must be ascertained by reference to some standard, he argued on the basis of an unending series of standards and refuted them. He rounded off his speech with the following: Listen, you Brahmins, standards are not applicable for ascertaining merits or demerits and so arriving at the truth. For erroneous standards are no good as tests. To start with, their correctness must be established. Other standards are required to check them. Are they in their turn infallible? Proceeding in this way, no finality can be reached. Therefore no tests are possible. Ascertainment of Truth being impossible without being tested, nothing can therefore be Truth. This enunciation itself cannot be true, nor the enunciator either. What then is the decision arrived at? That all are nothing, void. This too cannot be supported by reliable facts; hence, the statement that all are void ends in void also. Hearing his discourse, some of them were impressed by the force of Soonga’s logic and became scholiasts of the void. They got lost in the maze of their philosophy. The discriminating ones among the hearers placed Soonga’s arguments before their master and were enlightened by him. Thus they gained peace and happiness. Therefore, beware of arid polemics parading as logic. Use it in the manner in which the holy books have done. That way lies salvation.

The person narrating this story is a jivanmukta in this whose story is narrated by Lord Dattatreya himself to Lord Parasurama.

The Pramanas which were accepted by schools like Nyaya, vaiseshika, Mimamsa, Sankhya etc., were called āstika schools because they accept veda as authority and have different views regarding the ultimate truth. Also the whole Smritis, Puranas, Dharma Shastras came inorder to explain the heart of veda.

Kanchi Paramachaya HH Jagadguru Sri Chandrasekhara Saraswati explains as (Hindu Dharma - The Universal Way of Life)

The Puranas are the magnifying glass of the Vedas. The principles and rules of dharma that are briefly dealt with in the Vedas are enlarged or elaborated upon in them in the form of stories. (Magnifying Glass of the Vedas)

The Vedas are the root of this dharma, its fountainhead. But the rites and duties are not given in an orderly form in the Vedas, nor is the procedure for works laid down in detail. Of the Vedas that are infinite we have obtained only a very small part. And we do not comprehend fully the meaning of many of the passages even of this small part. The Dharmasutras (by Apastamba, Gautama and others) are terse statements and are so according to the very definition of the term "sutras". The dharmasastras (by Manu, Yagnavalkya, Parasara and others) are called Smrtis and are in verse and detail in treatment. Their basis, however, is constituted by the Vedas. The function of Dharmasastra is to analyse and explicate the sutras of Kalpa which have to some extent systematised the Vedic rules and injunctions. If Kalpa gives instructions about the constructions of the Vedic altar, of houses, etc, Dharmasastra provides a code of conduct embracing all human activities. "Smrti" is what is remembered. "Vismrti" is insanity. Manu observes :"There is Smrti for the Vedas in the form of notes. The sages who had a profound understanding of the Vedas have brought together the duties and rites (dharma and karma) mentioned in them in the form of notes and they constitute the Smrtis. They are written in a language that we can easily understand. Read them. They tell you about your in detail, the do's and don'ts, and how the rites are to be performed. " (Realising the Ideals of the Puranas)

The best testimony to the claim that the Smrtis are founded on the Vedas is provided by the words of mahakavi(great poet). The greatest of the mahakavis, Kalidasa, makes a reference to the Smrtis in his Raghuvamsam.

Tasyah khuranyasapavitrapamsum

Apamsulanam dhuri kirtaniya

Margam manusyesvaradharmapatni

Sruterivartham Smrtiranvagacchat

-Raghuvamsam, 2. 2

So Sudhaksina followed in the "hoof steps" of Nandini like the Smrtis following the meaning of the Vedas. Also, like the Smrtis not going in the entire way with the Vedas, she did not go all the distance with the cow. The idea is that the Smrtis do not repeat all that is said in the Vedas. They are "notes from memory", but they truthfully follow the Vedas in their meaning. They do not, of course, represent all thousands of mantras of the scriptures but, all the same, they tell us how to make use of the Vedas.

"Sudhaksina with her pure antah-karana followed her husband and, without deviating even a little, walked along the path of the dust raised by Nandini's hoofs". Having said so much, Kalidasa thought he must bring in a good simile for Sudhaksina following the cow dust and it occurred to him in a flash: "Like the Smrtis following faithfully the meaning of the Vedas. " (The Source of Smritis is the Vedas)

Also in Ribhu Gita, Lord Siva says

श्रुतिरियं कर्तृत्ववादं वदत् meaning śruti (vedas) alone are the authority and guidance for all.

So, It is very difficult to prove the authority of the vedas as stated by Sringeri Jagadguru. Also if the schools whose sole existence was possible just because they were āstika gave arguments for vedas' authority, then how valid are their arguments ?

So always Vedas are the pramana whose authority cannot be questioned (though it might sound harsh) and is difficult to be proved based on means like Pratyaksha, Anumana.

Hope this helps you.