Brihadaranyaka Upanishad 4.5.15

4.5.15

yatra hi dvaitamiva bhavati taditara itaraṃ paśyati taditara itaraṃ jighrati taditara itaram̐ rasa-yate taditara itaramabhivadati taditara itaram̐ śṛṇoti taditara itaraṃ manute taditara itaram̐ spṛśati taditara itaraṃ vijānāti yatra tvasya sarvamātmaivābhūttatkena kaṃ paśyettatkena kaṃ jighrettatkena kam̐ rasayettatkena kamabhivadettatkena kam̐ śṛṇuyāttatkena kaṃ manvīta tatkena kam̐ spṛśettatkena kaṃ vijānīyādyenedam̐ savaṃ vijānātitaṃ kena vijānīyātsa eṣa neti netyātmāgṛhmo na hi gṛhmate'śīryo na hi śīryate'saṅgo na hi sajyate'sito na vyathate na riṣyati vijñātāramare kena vijānīyādityuktānuśāsanāsi maitreyyetāvadare khalvamṛtatvamiti hoktvā yājñavalkyo vijahāra

Because when there is duality, as it were, then one sees something, one smells something, one tastes something, one speaks something, one hears something, one thinks something, one touches something, one knows something. But when to the knower of Brahman everything has become the Self, then what should one see and through what, what should one smell and through what, what should one taste and through what, what should one speak and through what, what should one hear and through what, what should one think and through what, what should one touch and through what, what should one know and through what? Through what should one know that owing to which all this is known? This self is That which has been described as 'Not this, not this.' It is imperceptible, for It is never perceived; undecaying, for It never decays; unattached, for It is never attached; unfettered—It never feels pain, and never suffers injury. Through what, O Maitreyi, should one know the knower? So you have got the instruction, Maitreyi. This much indeed is (the means of) immortality, my dear.' Saying this Yājñavalkya left home.

4.5.15.p1

iti caturthādhyāye pañcamaṃ brāhmaṇam