The passage "to know the unknowable" is a smooth and quite correct translation, but there is a deeper sense in it and it certainly should not be interpreted in the sense of agnosticism. A strictly correct literal translation should read "know the not-knowing," which
means "be familiar with that state of mind where knowing (the noetic faculty) is not the medium of our mental life." It is an expression of Lao-tze's mysticism in which the attitude of heart is considered superior to comprehension, and seems to involve what European mystics call intuition and what is characterized by St. Paul as the "peace that passeth understanding." We can retain the translation "unknowable" if it is understood in this sense, not as anything incomprehensible, an x in cognition, but as a mental attitude, as the feeling of the ineffable.
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The connection between the first and second paragraphs consists in the idea that courage is sometimes successful and sometimes it brings harm. We do not know the reason why heaven sometimes dooms a hero. The word "doom," translated in the text "reject," reads in the Chinese "hate."