The Criterion of Multiple Attestation
If two sources that never talked to each other report the same thing, that's not a coincidence.
Grounded in the research on the criterion of multiple attestation
So what even is it?
It's a historian's filter. The idea: if a saying or event about Jesus shows up in several independent sources, it's more likely to go back to something that actually happened than something one author invented. One source can lie or embellish. Multiple sources that never coordinated, all landing on the same detail, are a lot harder to explain away. More independent witnesses, more confidence. That's the whole engine.
Who actually uses this?
Mainstream historians, not just apologists. F.C. Burkitt floated it in 1906 at the tail end of the first quest for the historical Jesus. Norman Perrin nailed it down as a formal tool in his 1967 book Rediscovering the Teaching of Jesus. It's now one of the standard 'criteria of authenticity' taught in secular New Testament scholarship, used by skeptics like Bart Ehrman and believers alike.