Brihadaranyaka Upanishad 2.4.13

2.4.13

sā hovāca maitreyyatraiva mā bhagavānamūmuhanna pretya saṃjñāstīti sa hovāca yājñavatkyo na vā are'haṃ mohaṃ bravīmyalaṃ vā ara idaṃ vijñānāya

Then Maitreyî said: 'Here thou hast bewildered me, Sir, when thou sayest that having departed, there is no more knowledge.'
But Yâgñavalkya replied: 'O Maitreyî, I say nothing that is bewildering. This is enough, O beloved, for wisdom.
'For when there is as it were duality, then one sees the other, one smells the other, one hears the other, one salutes the other, one perceives the other, one knows the other; but when the Self only is all this, how should he smell another, how should he see another, how should he hear another, how should he salute another, how should he perceive another, how should he know another? How should he know Him by whom he knows all this? How, O beloved, should he know (himself), the Knower?'