Kena Upanishad 2

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?'

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.

'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.

'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.

'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.'