BṚHADĀRAṆYAKA UPANISHAD · 4.3.23

यद् वै तन् न पश्यति पश्यन् वै तन् न पश्यति ।
न हि द्रष्टुर् दृष्टेर् विपरिलोपो विद्यते ऽविनाशित्वान् ।
न तु तद् द्वितीयम् अस्ति ततो ऽन्यद् विभक्तं यत् पश्येत् ॥

yad vai tan na paśyati paśyan vai tan na paśyati ।
na hi draṣṭur dṛṣṭer viparilopo vidyate ’vināśitvān ।
na tu tad dvitīyam asti tato ’nyad vibhaktaṃ yat paśyet ॥

'And when (it is said that) there (in the Sushupti) he does not see, yet he is seeing, though he does not see. For sight is inseparable from the seer, because it cannot perish. But there is then no second, nothing else different from him that he could see.

F. Max Müller · The Upanishads, Sacred Books of the East 15 (1884)