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The key to an answer is that witness itself makes or implies certain claims to validity -- specificallyvalidity—specifically, to be appropriate, credible, and fitting -- that fitting—that may or may not be valid claims, but that have to be valid if witness itself is the undertaking it purports to be. In other words, witness, by its own claims, subjects itself to a standard of judgment beyond itself by which it has to be measured and judged valid, else it is not really itself, or, at any rate, not wholly so.

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