The Upanishads, Part 1 (SBE01), by Max Müller, , at sacred-texts.com
1. The Teacher says: 'If thou thinkest I know it well, then thou knowest surely but little, what is that form of Brahman known, it may be, to thee 2?'
2. The Pupil says: 'I do not think I know it well, nor do I know that I do not know it. He
among us who knows this, he knows it, nor does he know that he does not know it 1.
3. 'He by whom it (Brahman) is not thought, by him it is thought; he by whom it is thought, knows it not. It is not understood by those who understand it, it is understood by those who do not understand it.
4. 'It is thought to be known (as if) by awakening, and (then) we obtain immortality indeed. By the Self we obtain strength, by knowledge we obtain immortality.
5. 'If a man know this here, that is the true (end of life); if he does not know this here, then there is great destruction (new births). The wise who have thought on all things (and recognised the Self in them) become immortal, when they have departed from this world.'
148:2 In order to obtain a verse, we must leave out the words tvam yad asya deveshv atha nu mîmâmsyam eva. They were probably inserted, as an excuse for the third khanda treating of the relation of Brahman to the Devas. There is considerable variety in the text, as handed down in the Sâma-veda and in the Atharva-veda, which shows that it has been tampered with. Daharam for dabhram may be the older reading, as synezesis occurs again and again in the Upanishads.
149:1 This verse has again been variously explained. I think the train of thought is this: We cannot know Brahman, as we know other objects, by referring them to a class and pointing out their differences. But, on the other hand, we do not know that we know him not, i. e. no one can assert that we know him not, for we want Brahman in order to know anything. He, therefore, who knows this double peculiarity of the knowledge of Brahman, he knows Brahman, as much as it can be known; and he does not know, nor can anybody prove it to him, that he does not know Brahman.