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Missionary proclamation was once understood as bringing the truth to others, and was therefore both legitimate and extremely important. For many today, the missionary enterprise is a matter of imposing our personal preferences and culturally conditioned prejudices upon others, and is therefore not only illegitimate but morally offensive.
Attributed to Confucius by Henry David Thoreau in "Economy" from Walden, collected in Thoreau: Political Writings(1996), pp. 27-28. (This quote — "To know that we know what we know, and that we do not know what we do not know, that is true knowledge." — is often attributed to Nicholas Copernicus, but I've been unable to find a citation bearing this out.)
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To know that we know what we know, and that we do not know what we do not know, that is true knowledge.