Add to ORKGNo question's solved until error's resolved. —Prospective proverb 1. Introduction. How do scientists know—and justify—that they have erred? The question virtually bristles with paradox. Error seems the very antithesis of knowledge. How could one justify such a "negative " discovery? Oddly perhaps, to know that a claim deemed right in one context is wrong requires justification. I focus here on this dimension of the scientific enterprise, the ascertaining of error, and its relation to the general problem of characterizing reliable knowledge