Mandukya Karika, verse 3.19


मायया भिद्यते ह्येतन्नान्यथाऽजं कथञ्चन ।
तत्त्वतो भिद्यमाने हि मर्त्यताममृतं व्रजेत् ॥ १९ ॥

māyayā bhidyate hyetannānyathā'jaṃ kathañcana |
tattvato bhidyamāne hi martyatāmamṛtaṃ vrajet || 19 ||

19. This unborn (changeless, non-dual Brahman) appears to undergo modification only on account of Māyā (illusion) and not otherwise. For, if this modification were real, the Immortal (Brahman) would become mortal.

Shankara Bhashya (commentary)

If duality1 were the effect of non-duality, then it could be contended that duality also, like the Advaita, is the Supreme Reality. In order to remove this doubt which may crop up in the minds of some, it is said that non-duality which is the Supreme Reality appears manifold through Māyā,2 like the one moon appearing as many to one with defective eye-sight and the rope appearing (to the deluded) as the snake, the water-line, etc. This manifold is not real, for Ātman is without any part. An object endowed with parts may be said to undergo modification by a change of its parts, as clay undergoes differentiation into pots, etc. Therefore the purport is that the changeless (unborn) Ātman which is without parts cannot, in any manner, admit of distinction excepting through Māyā or the illusion of the perceiver. If3 the appearance of manifoldness were real, then the Ātman, the ever-unborn and non-dual, which is, by its very nature, immortal would become mortal as though fire would become cold (which is an absurdity). The4 reversal of one’s own nature is not desired by any—as it is opposed to all means of proofs. Therefore the Reality—which is Ātman—changeless and unborn, appears to undergo a modification only through Māyā. Hence it follows that duality is not the ultimate Reality.