For the pure, ideal Bayesian, everything is "evidence". Given the probabilities that you currently assign to all possible statements about the world, when you observe that some statement P is true, you update all your probabilities in accordance with the mathematical rules.

If I then ask, "suppose I don't observe that P is true, only something suggesting that P is likely true?" the answer is that in that case I did not observe P. I observed something else Q. It is then the truth of Q that I should use to update my probabilities for P and everything else.

To elaborate rather than adding another answer:

For more human purposes, we can't treat everything as evidence because it's really hard to know what the implications are of every piece of raw data - even if those implications are perfectly deterministic, we haven't got the brain power to figure them all out. And we can sort of turn this argument around to see that when we

canfigure out some implications of a piece of raw data, then we can treat it as evidence.And so in everyday life we do the same thing we do when solving all other sorts of problems - we u... (read more)