Bhashya
ननु कथम् अन्यस्मिन्विदिते अन्यद्विदितं भवति ? नैषदोषः ; न हि आत्मव्यतिरेकेण अन्यत्किंचिदस्ति ; यद्यस्ति,न तद्विदितं स्यात् ; न त्वन्यदस्ति ; आत्मैव तु सर्वम् ;तस्मात् सर्वम् आत्मनि विदिते विदितं स्यात् ।कथं पुन-रात्मैव सर्वमित्येतत् श्रावयति-ब्रह्म ब्राह्मणजातिः तं पुरुषंपरादात् परादध्यात् पराकुर्यात् ; कम् ? यः अन्यत्रात्मनःआत्मस्वरूपव्यतिरेकेण- आत्मैव न भवतीयं ब्राह्मणजाति-रिति- तां यो वेद, तं परादध्यात् सा ब्राह्मणजातिः अनात्म-स्वरूपेण मां पश्यतीति ; परमात्मा हि सर्वेषामात्मा ।तथाक्षत्रं क्षत्रियजातिः, तथा लोकाः, देवाः, भूतानि, सर्वम् ।इदं ब्रह्मेति- यान्यनुक्रान्तानि तानि सर्वाणि, आत्मैव,यदयमात्मा- योऽयमात्मा द्रष्टव्यः श्रोतव्य इति प्रकृतः-यस्मात् आत्मनो जायते आत्मन्येव लीयत आत्ममयं चस्थितिकाले, आत्मव्यतिरेकेणाग्रहणात्, आत्मैव सर्वम् ॥
In plain words
Someone might object: if you know one thing, how does that make another thing known? It isn't a flaw at all. Nothing exists apart from the Self — if it did, knowing the Self wouldn't touch it. But nothing else exists. The Self alone is everything. So once you know the Self, everything is already known. But how is the Self really everything? Scripture itself says so. Take the word 'brahma' — it means the class of brāhmaṇas here, and it rejects a man: the one who imagines that this class stands apart from his own true nature as the Self. Whoever thinks of the brāhmaṇa community that way is himself cast off by it — as if that whole community said, 'he sees me as something other than himself.' Because the Supreme Self is the Self of everyone. The same holds for 'kṣatra,' the warrior class, and for the worlds, the gods, all creatures, everything. 'This is Brahman' — every single thing named is nothing but the Self. This Self, the very one that is to be realized and heard about, is what the teaching has been pointing to all along — because everything is born from the Self, dissolves back into the Self alone, and while it lasts is made of nothing but the Self. Nothing is ever found apart from it. The Self is everything.
Close to the Sanskrit
Objection: How, when one thing is known, does another thing come to be known? [Reply:] This is not a flaw. For nothing else exists apart from the Self; if anything did exist, it would not be known [merely through the Self being known]; but nothing else exists — the Self alone is everything. Therefore, when the Self is known, everything would be known. But how again is it that the Self alone is everything — this the scripture makes known: 'Brahman' means the class of brāhmaṇas; it rejects that man — whom? One who, apart from the Self, [thinks], through difference from the very nature of the Self, 'the Self alone is not this class of brāhmaṇas' — whoever knows that class in this way, him it rejects; that class of brāhmaṇas [says], 'he sees me in the form of a non-Self' — for the Supreme Self is indeed the Self of all. Likewise 'kṣatra' means the class of kṣatriyas; likewise the worlds, the gods, the creatures, everything. 'This is Brahman' — all those things enumerated are the Self alone; the [phrase] 'this Self,' which is this Self that is to be seen and to be heard of, is the one under discussion — because everything is born from the Self, dissolves in the Self alone, and during the time of its continuance consists of the Self, since nothing is ever apprehended apart from the Self, the Self alone is everything.