What would it take to give evidence that prayer ever works? Just one single example.
What would it take to give evidence that prayer never works? Analyzing every single prayer ever made and ever to be made.
Er,.... no.
You're confusing "evidence" and "proof." To
prove that prayer never works would indeed require analyizing every prayer ever made. Just to provide
evidence that it never works is much easier.
Think of it in a legal context:
"My lord, the Crown will clearly show evidence that the Defendant sold a patently ineffective substance with the intent to defraud..."
"Objection, my lord. To show evidence that coloured water is indeed patently ineffective as a cure for smallpox, the Crown would have to analyze every single case of smallpox ever treated with coloured water, including all future cases yet to be diagnosed!"
"Hmm. Upon careful consideration of the philosophical considerations raised by the Defendant's barrister, I hereby sentence said barrister to be beaten to death with rifle butts by a platoon of the Royal Marines, and may God have mercy upon his client's soul."
Having said this, there's lots of evidence, dating back at least to Galton, that prayer does in fact, not work.