यत्र हि द्वैतमिव भवति तदितर इतरं जिघ्रति तदितर इतरं पश्यति तदितर इतरं शृणोति तदितर इतरं जिघ्रति तदितर इतरमभिवदति तदितर इतरं मनुते तदितर इतरं विजानाति ।यत्र वास्य सर्वमात्मैवाभूत्तत्केन कं जिघ्रेत्तत्केन कं जिघ्रेत्तत्केन कं पश्येत्तत्केन कं शृणुयात्तत्केन कमभिवदेत्तत्केन कं मन्वीत तत्केन कं विजानीयात् ।येनेदं सर्वं विजानाति तं केन विजानीयात् ।विज्ञातारमरे केन विजानीयादिति ॥बृह. २,४.१४ ॥
In plain words
For where there is duality as it were, there one smells another, one sees another, one hears another, one speaks to another, one thinks of another, one knows another. But where all has become one's very Self, then with what should one smell whom? with what should one see whom? with what should one hear whom? with what should one speak to whom? with what should one think of whom? with what should one know whom? That by which one knows all this — with what should one know that? With what, my dear, should one know the knower?"
The commentators
Bṛhadāraṇyakopaniṣadbhāṣyam· bhāṣya
ज्ञानमुपपद्यते ; तस्मात् येन इदं सर्वं विजानाति, तं विज्ञा-तारं केन करणेन को वा अन्यः विजानीयात्- यदा तु पुनः परमार्थविवेकिनो ब्रह्मविदो विज्ञातैव केवलोऽद्वयो वर्तते, तं विज्ञातारं अरे केन विजानीयादिति ॥
In plain words
Knowing needs something to know -- so how could you ever turn around and know the Knower itself, the one by which everything else is known? No instrument reaches it, and there is no second knower standing outside it to see it. And once someone truly sees reality as it is, nothing is left but that Knower alone, without a second. Tell me, dear one -- by what could you possibly know that?
Close to the Sanskrit
Knowledge does not arise [without an object]; therefore, that by which one knows all this -- that Knower -- by what instrument, or what other [knower], could one know? But when, for the knower of Brahman who has discriminated the highest reality, only the Knower alone, non-dual, remains -- by what, O dear one, could one know that Knower?