Front Page Titles (by Subject) (SECOND KHAṆḌA) - The Thirteen Principal Upanishads
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(SECOND KHAṆḌA) - Misc (Upanishads), The Thirteen Principal Upanishads 
The Thirteen Principal Upanishads, translated from the Sanskrit with an outline of the philosophy of the Upanishads and an annotated bibliography, by Robert Ernest Hume (Oxford University Press, 1921).
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The paradox of Its inscrutability
9 (1). [Teacher:] If you think ‘I know well,’ only very slightly now do you know!—a form of Brahma!—what thereof is yourself, and what thereof is among the gods! So then it is to be pondered upon (mīmāṁsyam) indeed by you.
[Pupil:] I think it is known.3
10 (2). I think not ‘I know well’;
Yet I know not ‘I know not’!
He of us who knows It, knows It;
Yet he knows not ‘I know not.’
11 (3). [Teacher:]
It is conceived of by him by whom It is not conceived of.
He by whom It is conceived of, knows It not.
It is not understood by those who [say they] understand It.
It is understood by those who [say they] understand It not.
The value of knowledge of It
12 (4). When known by an awakening, It is conceived of;
Truly it is immortality one finds.
With the Soul (Ātman) one finds power1 ;
With knowledge one finds the immortal.
13 (5). If one have known [It] here, then there is truth.
If one have known [It] not here, great is the destruction (vinasṭi).2
Discerning [It] in every single being, the wise,
On departing from this world, become immortal.
[3 ]What has been translated as two sentences might also be construed as one sentence, still a part of the teacher’s reproof to the undiscerning pupil:—‘So then I think that what is “known” by you is [still] to be pondered upon indeed.’
[1 ]Perhaps ‘power [to know]; and with the knowledge [thus gained] one finds . . . .’
[2 ]With a slight variation this line is found also at Bṛih. 4. 4. 14 b.