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To see what is right and not to do it is want of courage, or of principle.

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When you know a thing, to hold that you know it, and when you do not know a thing, to allow that you do not know it - this is knowledge.

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If we don't know life, how can we know death?

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To know what you know and what you do not know, that is true knowledge.

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Real knowledge is to know the extent of one's ignorance.