Yajur Veda, Brihadaranyaka Upanishad II, IV-Yajnavalkya and Maitreyi

For when there is duality, as it were, then one smells another, one sees another, one hears another, one speaks to another, one thinks of another, one knows another. 

But when everything has become the Self, then what should one smell and through what, what should one see and through what, what should one hear and through what, what should one speak and through what, what should one think and through what, what should one know and through what?

 Through what should One know That owing to which all this is known-through what, my dear, should one know the Knower?