Mandukya Karika, verse 3.19

Sanskrit text, IAST transliteration and English translation

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.

Anandagiri Tika (glossary)

¹ Duality, etc.—For, the effect always partakes of the nature of the cause.

² Māyā—Māyā explains the appearance of the manifold consistently; not the Pariṇāmavāda (or the theory of actual transformation) adumbrated by the Sāṃkhyas.

³ If etc.—For, by changing into the universe, the non-dual Ātman which is admitted to be immortal, would undergo destruction and become mortal. A thing cannot retain its own nature while undergoing a change.

⁴ The reversal, etc.—One of the tests of Reality is that it never admits of any change of its innate nature. The non-dual Ātman being the Reality, can never really change into the dual universe. Therefore the act of creation or modification is an illusion. Hegel’s theory of logical necessity or Bradley’s Absolute somehow becoming the phenomena cannot be borne out by reason.