Mandukya Karika, verse 2.34


नाऽऽत्मभावेन नानेदं न स्वेनापि कथंचन ।
न पृथङ्नापृथक्किंचिद् इति तत्त्वविदो विदुः ॥ ३४ ॥

nā''tmabhāvena nānedaṃ na svenāpi kathaṃcana |
na pṛthaṅnāpṛthakkiṃcid iti tattvavido viduḥ || 34 ||

34. This manifold does not exist as identical with Ātman nor does it ever stand independent by itself. It is neither separate from Brahman nor is it non-separate. This is the statement of the wise.

Shankara Bhashya (commentary)

Why is non-duality called the highest bliss? One suffers from misery when one finds differences in the form of multiplicity, i.e., when one finds an object separate from another. For1 when this manifold of the universe with the entire relative phenomena consisting of Prāṇa, etc., imagined in the non-dual Ātman, the Ultimate Reality is realised to be identical with the Ātman, the Supreme Reality, then alone multiplicity ceases to exist, i.e., Prāṇa, etc., do not appear to be separate from Ātman. It2 is just like the snake that is imagined (to be separate from the rope) but that does no longer remain as such when its true nature is known with the help of a light to be nothing but the rope. This manifold (Idam) does never really exist as it appears to be, that is to say, in the forms of Prāṇa, etc., because3 it is imaginary just like the snake seen in the place of the rope. Therefore different objects, such as Prāṇa, etc., do not exist as separate from one other as a buffalo appears to be separate from a horse. The idea of separation being unreal, there is nothing which exists as separate from an object of the same nature or from other objects (of different nature). The Brāhmaṇas, i.e., the Knowers of Self, know this4 to be the essence of the Ultimate Reality. Therefore the implication of the verse is that non-duality alone, on account of the absence of any cause that may bring about misery, is verily the (highest) bliss.