There are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.
有已知的已知。这些事情我们都知道，我们都知道。有已知的未知。也就是说，有些事情我们知道我们不知道。但也有未知的未知。有些事情我们不知道，我们不知道。
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