Bhashya
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?