Mandukya Karika, verse 1.18


विकल्पो विनिवर्तेत कल्पितो यदि केनचित् ।
उपदेशादयं वादो ज्ञाते द्वैतं न विद्यते ॥ १८ ॥

vikalpo vinivarteta kalpito yadi kenacit |
upadeśādayaṃ vādo jñāte dvaitaṃ na vidyate || 18 ||

18. If anyone has ever imagined the manifold ideas (such for instance as the teacher, the taught, and the scripture), they might disappear. This explanation is for the purpose of teaching. Duality (implied in explanation) ceases to exist when the Highest Truth is known.

Shankara Bhashya (commentary)

(Objection)—How1 could (duality implied in) ideas such as the teacher, the taught and the scripture disappear? (Reply)—This is thus explained. If2 such ideas had ever been imagined by someone then they might be supposed to disappear. As the manifold is like the illusion (conjured up by the magician or) of the snake in the rope, so3 also are the ideas of the teacher, etc. These ideas, namely, the ideas of teacher, taught, and scripture are for4 the purpose of teaching which are (therefore appear) true till one realises the Highest Truth. But duality does not exist when one, as a result of the teaching, attains knowledge, i.e., realises the Highest Reality.