For this further reason, one should not on the strength of mere logic challenge something that has to be known from the Vedas. For reasoning that has no Vedic foundation and springs from the mere imagination of persons, lacks conclusiveness. For man’s conjecture has no limit. Thus it is seen that an argument discovered by adepts with great effort is falsified by other adepts; and an argument hit upon by the latter is proved to be hollow by still others. So nobody can rely on any argument to be conclusive for human intellect differs. If, however, the reasoning of someone having wide fame, say for instance, Kapila or someone else, be relied on under the belief that this must be conclusive, even so it surely remains inconclusive, inasmuch as people , whose greatness is well recognised and who are the initiators of scriptures (or schools of thought) - for instance, Kapila, Kanada and others - are seen to hold divergent views.(from Brahma-Sutra-Bhasya II.i.11)
Speaking of Brahman Sankara writes:
So what need has one to argue that the nature of Brahman whose power is beyond all thought, cannot be ascertained unless it be through the Vedas? So also has it been said by an author of a Purana, “Do not bring those things within the range of argumentation which are beyond thought. The nature of a thing beyond thought consists of its being other than the things within nature.” Hence a supersensuous thing is truly known from the Vedic source alone.(from B.S.B. II.i.27)
This is indeed contrary to the Scholastic tradition particularly that of Sankara’s opposite number Thomas Aquinas and his latter day followers. Other Christian denominations demur and the adherence to the Ontological Argument that has seemed supremely rational to some great minds is now generally discounted which indicates that the feeling of certainty is a mirage.
Sankara does not leave aside all reasoning in religion.
And when there is any divergence as regards interpretation of Vedic passages, it is through reasoning, meant for the determination of the meaning of sentences, that false interpretations are discarded and the proper import is determined.(from B.S.B. II.i.11)
The Vedas are not determinant of knowledge which is accessible through conventional means. Even if the Vedas were to declare that fire is not hot or water does not wet, this would not make it true. However on matters which are supersensuous the Vedas are authoritative and regarded as a pramana or reliable means of knowledge. The Mahavakas or great sayings such as Tat Tvam Asi or Aham Brahmasmi respectively ‘that thou art’ and ‘I am that Brahman’ are the key utterances. There are five in all. It is not as though these mahavakas could be experientially ratified for then they would not be beyond the senses, but that they are the metaphysical ground which makes experience as such possible. This is where the Advaitic concept of the non-duality of Subject and Object that is realized in experience comes in. The notion of realization is not the dawning of an awareness as in normal usage rather it is the condition that makes experience itself possible. Certain of the aporiai of natural reason as Sankara treats of them in his premable to the B.S.B. are resolved by the concept of non-duality, though the understanding of that extremely condensed passage is contested. What is the real meaning of the well known snake/rope analogy used therein? More anon.