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.