Mandukya Karika, verse 3.18


अद्वैतं परमार्थो हि द्वैतं तद्भेद उच्यते ।
तेषाम् उभयथा द्वैतं तेनायं न विरुद्ध्यते ॥ १८ ॥

advaitaṃ paramārtho hi dvaitaṃ tadbheda ucyate |
teṣām ubhayathā dvaitaṃ tenāyaṃ na viruddhyate || 18 ||

18. As non-duality is the ultimate Reality, therefore duality is said to be its effect (Kārya or Bheda). The dualists perceive duality either way (i.e., both in the Absolute and in the phenomena). Therefore the non-dual position does not conflict with the dualist’s position.

Shankara Bhashya (commentary)

How is it that the non-dualist does not conflict with the dualist? The reason is thus stated:—As1 nonduality is the ultimate Reality, therefore duality or multiplicity is only its effect. The Scriptural passages such as, “He is one and without a second”, “He created fire”, etc., support this view. It2 is further borne out by reason as duality is not perceived in the states of swoon, deep sleep or trance (samādhi), in the absence of the activity of the mind. Therefore duality is said to be the effect of non-duality. But the dualists perceive duality alone either3 way, that is, from both the absolute and the relative standpoints. As duality is perceived only by the deluded and non-duality by us who are enlightened,4 therefore our view does not clash with their views. For, the Scripture also says, “Indra (the Supreme Lord) created all these diverse forms through Māyā”, “There exists nothing like duality”. It5 is like the case of a man on a spirited elephant, who knows that none can oppose him, but who yet does not drive his beast upon a lunatic who though standing on the ground, shouts at the former, “I am also on an elephant, drive your beast on me”. Therefore from the standpoint of Reality, the Knower of Brahman is the very self of (even) the dualists. Hence, our, viz., the non-dualistic view does not clash with other views. It may be asked in view of the differences between the dualistic and the non-dualistic views, how it can be said that the latter does not find any contradiction with the former. The text of the Kārikā gives the reply. It says that the so-called duality does not exist at all. Whatever exists is non-dual Brahman alone. Therefore the non-dualist cannot quarrel with a thing which is ultimately non-existent.