Is it superfluous to say "empirical evidence" instead of just "evidence"? How are those two different? Is there such a thing as evidence that is not empirical?
It isn't superfluous, but it might be unnecessary. People do tend to mean empirical evidence when they talk about evidence. Empirical evidence is the evidence of the senses, of direct observation or measurement. Compare that to rational evidence, which is evidence that is the result of deduction or other reasoning, or anecdotal evidence which comes from personal testimony (which may be reliable or not).
For example, here we have Bob, and a dead man with a knife in his back.
- The knife has bloody fingerprints on it.
- The blood is tested and found to be the dead man's blood.
- The bloody fingerprints match Bob's fingerprints.
Individually, these are all empirical evidence. Together they justify the claim that Bob handled the knife, after touching the man's blood. They might also be referred to as "forensic evidence," meaning (empirical) evidence gathered or used in the scientific investigation of crime.
- Bob owns a knife.
- Bob's knife is missing.
- The knife in the dead man's back is the same kind of knife as Bob's missing knife.
Together, these form rational or circumstantial evidence that justifies the claim that the knife in the dead mans' back is Bob's knife.
- Sue says her mom used to say that someone who owns knife and isn't a cook is probably a murderer.
- Jack says he's never seen Bob chop a vegetable in his life.
- Bob's neighbors say he was "the quiet type" and "you know how that is."
These statements are anecdotal evidence that Bob is probably a murder. As is often the case, this evidence should be treated with skepticism unless supported by some other kind of more reliable evidence.
A good example would be:
A workplace has loud equipment working in it. This poses a risk to the workers. There are prescribed maximum levels of noise beyond which it has been proven to be damaging to hearing.
There is empirical evidence (through medical observation) that exposure to 85 decibels is damaging.
Manager X arranges for noise surveys (measured using detecting equipment) of the workplace. The results range from 65 to 75 decibels. He has empirical evidence that the legal exposure limit of 85 decibels is not being exceeded.