Sage Sankara :~ The seeker has to arrive at the truth of Non-Dualism by pure reason.+

Reasoning

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. 

Bhagavad Gita: ~ Reason as a means to reality. (Chap.18 verse 37)

In chap. 10 of in Bhagavad Gita, Krishna says: ~ I cannot save you, but I can give you Buddhi (reason). (chap. 10)

In chap. 10 of in Bhagavad Gita, Krishna says: ~ "I have given you the most secret teaching, now reflect over it all" Krishna plainly says reflect, think. (Verse 63 of Chap. 18)

Mundaka Upanishad says: ~  Both states are harmful and take you away from the path of inquiry into the truth. (Page 234)

Ashtavakra Gita: ~ 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. (Page~ 224). 

Brihad Upanishads:~ To see Him whole mystics may have similar ideas of the "universe in a dew-drop," but Advaitic truth demands proof. (Page 78) 

Upanishad says: ~ Atman, the Self is known by Reason alone, by the sharpened and purified intellect. Yet still, people worship Yoga and mysticism as the sole means of attaining Atman. 

The universe in which you exist is an illusion and there is no substance in its reality. Self-knowledge or Brahma knowledge is the end of all egocentric knowledge. Egocentric knowledge is dualistic knowledge, whereas Self-knowledge is Soulcentric knowledge. 

Advaitic wisdom is nothing to do with orthodox Advaita, which is dualistic. Orthodox Advaita is a sect, which blindly accepts and follows dogmas and superstitions. 

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

Nothing has to be accepted as truth without verification.  

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 for this idea of divine causality and can, therefore, dispense with Sruti and arrive at the truth of Non-Dualism by pure reason. 

Sage Sankara is the only sage who has final authority on the Advaitic truth. The Advaitic truth is rational truth and scientific truth without dogma. :~Santthosh Kumaar