If a person testifies he saw ghosts - is he a less reliable witness because he is biased in believing in them?
Well, this is kind of an odd case, since his very testimony assumes he believes in ghosts. That is, you don’t add any information to the testimony by telling me that he believes in ghosts. If he didn’t believe in ghosts, he wouldn’t have described his experience with those words.
(And I still don’t see what the word “biased” is doing here. Again, it appears to me to be a way of describing a belief that you don’t agree with).
There are various things we should say about this case:
(1) What, precisely, did this person witness? A white light? Something moving? A thick fog? A noise? Let’s get a more precise description, since “ghost” covers so many possibilities. It’s a vague and loaded descriptor.
(2) If the person witnessed something, it has to be something that can interact with physical matter—e.g., it has to reflect light, or cause vibrations in the air, or move objects. That is, it must be at least partly physical. So we can start looking there. (Assuming, of course, that this wasn’t some species of hallucination; something we cannot completely rule out, even with very reliable witnesses).
Insofar as you’re discussing reliable testimony, it certainly seems to me that the reliability of any given witness has to be gauged without reference to the specific case in question. That is, if you want to know whether this person is reliable, you have to look at everything else that he’s witnessed, and not this case in particular. That will give you a global indicator of his reliability, which you can then put to this particular case.
But even if he’s an extremely reliable witness generally, that will simply establish that he saw something (e.g., a fuzzy light), not that he saw a ghost. That he saw “a ghost” is to say more than he would have evidence for. In order to establish that he saw a ghost we would need to know that the existence of ghosts was a live possibility—we would need to have evidence for life after death, for the spooky interaction of souls and material objects, etc. Were we to gather all that evidence and marshall it into a theory that supported the existence of ghosts, then depending on the case, we might say that this witness was reliable in this case. But without that background evidence (which anyhow, as I’ve said, we completely lack), we can say that although this witness may well be extremely reliable, he has misdescribed his experience in this case.