GH has recently been on a kick about the subjectivity of evidence since what one person might consider to be convincing evidence, another person could easily not. Does that mean that no standards exist? Are our considerations about the quality of evidence basically irrational?
I don't think so. We form our conceptions on the quality of types of evidence by our experience in how often certain types of evidence lead to a confirmation of truthfulness and accuracy.
For example, suppose you get hearsay evidence from an emotional witness that is self-serving and harms the opposing side. How likely is this reliable? Well, we can reach back in our memories and remind ourselves how often people lie for their own good and to harm those they don't like. How often has such a quality of evidence been consistent with a confirmed truth? Very rarely, I would say.
Now, suppose we were talking about physical evidence. Fingerprints, DNA, security cameras, etc. If you have that kind of evidence, how often is the claim bourne out as correct? Very often, I think. Probably above 95% of the time.
How often is circumstantial evidence correlated with truth? Some of the time. Not as much as physical evidence, but still far better than self-serving hearsay.
How about simple witness claims? Anecdotal evidence? More rare than you might think. Eyewitnesses have been found to be much less reliable than was once thought. The mind can do tricks to our memories.
I'd love to see some statistical analysis done to see how often certain types of evidence correlate with the truthfulness of the claims they attempt to support. Might be fascinating and then we could get real numbers as a standard for the quality of evidence.