Sage Sankara's system of Advaita does not need the support of any scripture or revelation like the Vedas.+

Advaitic view~

Sage Sankara's system of Advaita does not need the support of any Scripture or Revelation like the Vedas. The Srutis may all disappear, yet will his school stand.  Since it is based, not upon the varying theological fancies, which are as numerous as the sands of the sea, but upon reason, the common heritage of all mankind, irrespective of color or creed or clime.

Sage Sankara:~ 'Like a servant who carries a lamp in front of you to find your way, and you have found it, so becomes the Veda to that person. What is the Veda? ~ utterances of those who have known the Truth. Here is one who has known the Truth; why should he or she depend upon the Veda further? Actual realization takes you beyond books. At a certain stage, books become a botheration. The Upanishad itself says that the 'words are only so much distraction for such minds.

Sage Sankara says: ~ ‘What is accepted without a proper inquiry will not lead to the final Goal. (Commentaries on Vedanta Sutra)  

Without verification, nothing has to be accepted as truth.

Why Sage Sankara himself says: ~ VC 59. The study of the Scriptures is useless so long as the highest Truth is unknown, and it is equally useless when the highest Truth has already been known.

60. The Scriptures, consisting of many words are a dense forest that merely causes the mind to ramble. Hence men of wisdom should earnestly set about knowing the true nature of the Self.

61. For one who has been bitten by the serpent of Ignorance, the only remedy is the knowledge of Brahman. Of what avail are the Vedas and (other) Scriptures, Mantras (sacred formulae) and medicines to such a one?

It is not that one should pore over the ancient scriptures.  There is no need to study first, and then realize. One has to realize first, then only he will know ‘what is the truth’ and ‘what is untruth’. One has to make his discoveries through the process of rational thinking.

That is why Sage Sankara says:~ V C65- As a treasure hidden underground requires (for its extraction) competent instruction, excavation, the removal of stones and other such things lying above it and (finally) grasping, but never comes out by being (merely) called out by name, so the transparent Truth of the Self, which is hidden by Maya and its effects, is to be attained through the instructions of a knower of Brahman, followed by reflection, meditation and so forth, but not through perverted arguments.

Sage Sankara says:~ The scriptures dealing with rituals, rewards are therefore addressed to an ignorant person. Thus, the rituals are meant for ignorant people.

Sage Sankara: ~ "Though I wear these robes of a Sanyasin, it is only for the sake of bread."

~ This shows he was wearing the religious robe only for the sake of bread." Thus, it means those who are wearing religious robe for the sake of bread.

All the rituals based on the false belief of Gods will not yield any fruits and they are meant for the ignorant populace who are unable to grasp the God beyond the form, time and space.

One of Sage Sankara’s missions was to wean people away from a ritualistic approach advocated by Mimamsakas and to project wisdom (jnana) as the means of liberation in the light of Upanishad teachings.

Sage Sankara criticized severely the ritualistic attitude and those who advocated such practices. However, the Orthodox texts that combined rituals with wisdom (jnana_karma_samucchaya) more in favor of the Mimamsaka position came into vogue, projecting Sage  Sankara as the rallying force of the doctrine.

That is why Sage Sankara:~ (11) As regards the rituals, Sage Sankara says, the person who performs rituals and aspires for rewards will view himself in terms of the caste into which he is born, his age, the stage of his life, his standing in society, etc. In addition, he is required to perform rituals all through his life. However, the 'Self' has none of those attributes or tags. Hence, the person who superimposes all those attributes on the changeless, eternal Self and identifies the Self with the body is a confusing one for the other; and is, therefore, an ignorant person. The scriptures dealing with rituals, rewards, etc. are therefore addressed to an ignorant person.  -Adhyasa Bhashya

Sage Sankara:~ (11.1) This ignorance (mistaking the body for Self) brings in its wake a desire for the well-being of the body, aversion for its disease or discomfort, fear of its destruction, and thus a host of miseries(anartha).This anartha is caused by projecting karthvya(“doer” sense) and bhokthavya (object) on the Atman. Sage Sankara calls this adhyasa. The scriptures dealing with rituals, rewards, etc. are, therefore, he says, addressed to an ignorant person. -Adhyasa Bhashya

Sage Sankara:~ (11.2) In short, person who engages in rituals with the notion “I am an agent, doer, thinker”, according to Sage Sankara, is ignorant, as his behavior implies a distinct, separate doer/agent/knower ; and an object that is to be done/achieved/known. That duality is Avidya, an error that can be removed by Vidya.  -Adhyasa Bhashya

Sage Sankara: ~ (12) Sage Sankara affirming his belief in one eternal unchanging reality (Brahman) and the illusion of plurality, drives home the point that Upanishads deal not with rituals but with the knowledge of the Absolute (Brahma Vidya) and the Upanishads give us an insight into the essential nature of the Self which is identical with the Absolute, the Brahman. -Adhyasa Bhashya

No conceptual God can exist, apart from the consciousness.  People are not aware of the fact that there is no individual God can exist, apart from Soul, which is in the form of consciousness. Thus the Soul or consciousness is the Self.   If there is no consciousness, then there is no-body, no ego, no universe, no religion, and no conceptual God. 

The tenet of Nirguna Brahman is true for  Sage Sankara, not because it is taught by the Sruti, but because it is based on the realization though it is also supported by the Sruti... The Advaitin knows that a legitimate doubt may have here to arise.  The Rishis may have truly spoken, but they may have been deluded themselves.  How are we certain that what the Rishis cognized is the Reality or Truth?  This can be proved according to the Advaita, only by the realization of truth beyond the form, time and space.

And also:~

Again, in the absence of this realization, Nirguna Brahman as an object of thought is mere sound without sense. To one who has not seen a penguin, for instance, the word has no meaning ... Of what use, then, is such Sruti to him?  Similarly, common sense tells the Advaitin that the meaning of the Sruti and especially where there are conflicting interpretations is made out by means of reasoning based upon the authority of realization, which is final.

Thus, the reason comes into play between Sruti and realization corroborating the data of intuition with those of the revealed texts.

But reason also permits discrimination between the different possible experiences, for, in an a priori astonishing fashion:

Realization  ... can reveal not two, but twenty thousand conflicting experiences.  And the business of the wise is to sift the ultimate truth from out of all these ... The Advaitin rejects nothing.  All human experiences are his data.  He tests all by reason.

The only Advaita can reply: it is the witness, the Seer. The Buddhists are in error in regards to the finite ego as illusory, and as having nothing more behind it: but they would have been perfectly correct in such outlook had they added the notion of the witness. How is it that Skandhas come together and compose the ego? Who sees them come and go? It is the witness, the Atman, and this lack Vedanta supplies in the seer and seen and reason Analysis. When they say that the mind comes and goes they are forgetting that there must be another part of the mind as consciousness which notices it and which tells them of this disappearance and appearance. All their misunderstandings arise from the fact that Buddha refused to discuss the ultimate questions. When Buddhism degenerates into Nihilism Advitin refutes it (See Manduka P.281).

The truth of a single reality within or underlying the illusory ego is all-important and without it Buddhism becomes fallacious.

Vedanta admits the transitoriness and evanescence of thoughts just like Buddhism, but not of the Mind which observes this transitoriness and knows it.

Manduka Upanishad:~  Buddhists borrowed from Upanishads because they were Indians. The Vedantins did not need to borrow from Buddhism therefore (see P.396 v.99)

Bhagavan Buddha taught the illusoriness of ego but did not go further, probably because he thought the world could not understand the higher truth. Hence, followers go with him to that point of his and then deny the Vedantic doctrine of one supreme reality when the Bhagavan Buddha himself neither denied nor advocated it. Anyway, the refutation of his followers is to ask them “What is it that is aware of the ego's illusoriness?" There must be something that tells you that. That something is the  Drik, and if you say this Drik itself may be illusory, coming and going, still there must be something non-transient i.e.permanent, to tell you this.

Bhagavan Buddha's teachings that all life is misery belongs to the relative standpoint only. For you cannot form any idea of misery without contrasting it with its opposite, happiness. The two will always go together.

Bhagavan Buddha taught the goal of cessation of misery, i.e. peace, but took care not to discuss the ultimate standpoint for then he would have had to go above the heads of the people and tell them that misery itself was only an idea, that peace even was an idea (for it contrasted with peacelessness). That the doctrine he gave out was a limited one, is evident because he inculcated compassion. Why should a Buddhist sage practice pity? There is no reason for it.

Remember:~

Advaita is the next step higher than Buddhism because it gives the missing reason, viz. unity, non-difference from others, and because it explains that it used the concept of removing the sufferings of others, of lifting them up to happiness, only as we use one thorn to pick out another, afterward throw both away. Similarly, Advaita discards both concepts of misery and happiness in the ultimate standpoint of non-duality, which is indescribable.

Buddhists say that a thing exists only for a moment, and if that thing has still got some of the substance from which it was produced how then can they deny that its cause is continuing in the effect; hence its existence is more than a moment. Vedanta is concerned with whether it is one and the same thing which has come into being or has it come out of nothing.

Even the Sunyavada ultimate of the "void" is really a breath, and therefore an imagination and not truth.

Bhagavan Buddha was a Gnani, not his followers who follow Buddhism. Bhagavan Buddha's wisdom is lost in the diverse ideology of Buddhism. those who follow Hinduism or Buddhism will never be able to grasp the Advatic treasure hidden by the great Sages of truth. 

 Bhagavan Buddha as a constructive worker committed an error in failing to give the masses a religion, something tangible they could grasp, something materialistic, if symbolic that their limited intellect could take hold of, in addition to his ethics and philosophy. Here Sage Sankara was wiser and gave the religion, rituals -to the ignorant populace, as well as Advaitic wisdom to the serious seekers of truth.

BhagvanBuddha gave as the central feature of his doctrine the great law of Karma in order to reiterate its ethical meaning. He did more good in this to uplift the people than the ritualists.

Tibetan and Chinese Buddhists who say that there are many Bhagavan Buddhas living in spirit bodies and helping our earth from the spiritual world are still in the sphere of religious illusion, not the ultimate truth. Their statements are wrong. Every sage realizes that the only way to help mankind is to come down amongst them, for which he must necessarily take on flesh-body. When people are suffering how can he relieve their suffering unless he appears amongst them? When people are suffering how can he feed them from an unseen world whether their struggle is for material bread or for spiritual truth? No! He must be here actually in the flesh. It is impossible to help them in any other way and all talk of Shiva living on Mount Kailas in the spiritual body or Bhagavan Buddha in Nirmanakaya, invisible body belongs to the realm of delusion or self-deception.

Dalai Lama said: ~ Buddhism need not be the best religion though it is most scientific and religion and inquisitive. But Buddhism has no answer to certain questions like the existence of Atama (Soul) and rebirth. Dali Lama said that as an individual he believes in rebirth as he had come across a few cases of rebirth. Modern science, Dalai Lama hoped would unearth the mystery behind the rebirth. (In DH –dec-212009-Gulbarga)

The Upanishads have the answer to the existence of the Atama.

Brihadaranyaka Upanishad: ~ Brahman (God in truth) is in the form of the Athma, and it is indeed Athma itself.

Advaita is the next step higher than Buddhism because it gives the missing reason, viz. unity, non-difference from others, and because it explains that it used the concept of removing the sufferings of others, of lifting them up to happiness, only as we use one thorn to pick out another, afterward throw both away. Similarly, Advaita discards both concepts of misery and happiness in the ultimate standpoint of non-duality, which is indescribable.

Sage Sankara disagrees with Buddhists who say, there is nothing - a nonentity. Sage  Sankara believes there is some reality, even though things are not what they appear to be. If one knows the truth, he will know what to do to find inspiration for action. The seeker of truth‘s subject is to know what is it that is Real.

Buddhism says: all things are illusory and nothing exists. However, Advaita avers that it is not so. It says that the universe, of course, is illusory, but there is Brahman (Consciousness), that exists forming the very substratum of all things (illusion or universe). : ~ Santthosh Kumaar