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Verifier
Something whose job is to try to prove the work wrong.
A test, a rule, a second model, a person. What makes a verifier useful is independence: it has to be able to fail the work for reasons the thing that produced the work did not consider. Adding more verifiers that all think the same way buys very little, because identical critics are blind in identical places.
Why it matters
Diversity of verifier matters more than number of verifiers.