Sage Sankara believed that those of superior arrive at the truth of Non-Dualism by pure reason.+

Ashtavakra Gita page 224:~ It is not the absence of Buddhi (reason) that can grasp Advaita but the man who possesses the highest intellect. Brains are necessary. Such a man, by merely hearing the truth mentioned will know it.

Sage Sankara believed that those of superior intelligence, have no need of this idea of divine causality and can, therefore, dispense with Sruti and arrive at the truth of Non-Dualism by pure reason. 

Sage Goudpada: ~ To establish the truth of Nonduality by sheer reasoning alone. He begins by defining "What is real?" "What is unreal?" etc, because that is the right way to discuss or teach. People must first know what they are talking about. (Manduka Karika)

The reasoning is interpretable in two ways. The egocentric interpretation is to apply it only to the practical purpose (waking experience). The Soulcentric interpretation is to apply it to the three states. The latter leads to a final settlement of the problems because it takes all data into consideration.

Sage Sankara founded his Advaita Vedanta either on reason independent of Sruti or on Sruti confirmed by reason." 

Sage Sankara's commentary on the Mandukya Upanishad, II, 1:  This (the unreality of duality) is borne out by the Srutis ... But it is possible also to show the unreality of the object world even from pure reasoning, and this second chapter is undertaken for that purpose.

Sage Sankara himself had often said that his philosophy was based on Sruti or revealed scripture.  This may be because Sage Sankara addressed the ordinary man, who finds security in the idea of causality and thus in the idea of God—and Revelation is indispensable to prove the latter.  He believed that those of superior intelligence, have no need of this idea of divine causality and can, therefore, dispense with Sruti and arrive at the truth of Non-Dualism by pure reason. 

Sage Sankara, in debates with Buddhists and others who did not recognize the authority of the Vedas, had been obliged to prove the truth of Advaita through reason alone. 

Manduka Upanishad, which appealed to reason to the exclusion of Revelation. 

The experience implies duality.  The ‘I’ is present in the form of the Mind. The ‘Mind’ is an experience, which contains everyone and everything and the universe. Since one limits the mind to the physical entity, he forgets the fact that his body and the universe are an experience, within waking or dream experience. It is impossible to unfold the mystery of his true existence without realizing the fact that, the Self is not physical but the Self the formless Soul, or the Spirit.

If the Self is not physical then it is erroneous to judge and conclude the truth on the base of the physical self (ego). The truth has to be judged on the base of the Soul, the innermost   ‘Self, which is present in the form of consciousness. 

Remember:~

Until and unless one rectifies the reasoning base from the form (ego) to formless (Soul) the truth will not be revealed.

The reason is the common ground for the whole of humanity in the modern age, whereas the appeal to scriptural relations reaches only the sects.  Those who argue that truth is only in their religion are vain logicians, depending on mere ideas, speculations,  and imaginations.

Soulcentric Reasoning takes one to accuracy and guides one to the end because the truth will prevail when the illusion goes. One should not merely imagine and speculate things and assume something as truth, but verify them by the exercise of reasoned judgment.

Deeper thinking is essential, in the pursuit of truth. It is the man with sharpness and reason who gets on,  on this mental journey that brings success.

The truth which has no basis is usually religion or a mere catalog of ignorant opinions; people prefer it because it can be got more easily.  The Religious philosophies are based on the physical Self (waking entity or ego) being worthless from the ultimate standpoint.

Remember:~

The truth is incomplete and inaccurate without Soulcentric reasoning. Truth means certain and exact knowledge. The science which limits it to its practical applications such as modern machines is wrong because science is only dualistic wisdom. There is no need to learn any practical science to get the ultimate truth. Science is very helpful to give more comfort to mankind. But the ultimate truth is beyond the physical-based science.   But one has to have a certain knowledge of the fact that, the Self is not physical before indulging in the pursuit of truth.

Only through Soulcentric reasoning gets the exactness and certitude of knowledge, therefore, he acquires the Self-Knowledge or Brahma Gnana or Atma Gnana. When one has exact and certain knowledge of un-contradictable truth he can utilize it for guiding his fellow seekers.  The people who assume without any proof, have an untrained reason.

Many People adopt the attitude that what they know is the truth. And what others say is false. This attitude makes them not verify anything other than what they know.   One needs to be rational, not merely logical. Logic has its value only on the physical plane.

As one advances towards a spiritual plane he sees the logic underlying experience and becomes more rational. His reasoning is two-fold— on the dualistic perspective and on the nondualistic perspective. :~Santthosh Kumaar