Existing knowledge already has evidence
When a new claim contradicts a well-supported explanation, it must address the evidence that made the existing view credible.
Rare events have lower prior probability
Before examining new evidence, an event known to be extremely rare is less likely than a common error, misunderstanding or ordinary cause.
Human error is often more probable
Misidentification, memory distortion, fraud, coincidence and measurement failure are well-documented and may explain surprising reports.
Stronger does not mean impossible evidence
The standard should not be designed so that no evidence could ever succeed. It should require reliable, repeatable and independently verified support.
Multiple weak reports may remain weak
A large number of anecdotes can show that people have similar experiences, but may not establish the extraordinary explanation attributed to them.
Extraordinary evidence should discriminate between explanations
Good evidence should be more likely if the extraordinary claim is true than if ordinary alternatives are true.
Evidence notes
Strong support may include controlled observation, independent replication, reliable records, verified predictions and evidence that rules out ordinary error, fraud and alternative causes.
Ethical questions
What established evidence would this claim require us to revise?
Are ordinary explanations less likely than the extraordinary one?
What observation would distinguish the claim from error or coincidence?
Conclusion
Extraordinary claims need stronger evidence because ordinary mistakes and explanations are usually more probable. This principle protects inquiry without declaring unusual discoveries impossible.