If ghosts exist does this mean that mediums aren't fake because they claim to speak with the dead?
If Rolls-Royce's exist does this mean that I'm not lying because I claim to own six of them?
To put this rebuttal in a more generalized form that can be used in lots of other cases...
Questions or statements in "if-then" form are often syllogisms (things that actually have at least two inputs to yield one output when combined) with one of the inputs hidden. The proper form would be "if-if-then", such as "if this is the case
and that other thing is also the case, then here's a conclusion (whether being asserted or asked about)", but one of the "if" parts is missing.
So the useful general rule when encountering or thinking up "if-then" stuff is to ask what other unspoken "if" would be required to get to that "then".
To apply that to the specific case you brought up, getting from "ghosts are real" to "mediums' claims about them are true" requires another piece, such as "
if something is real, then people's claims about it are true". That kind or premise is built in to the question as if it were automatic, but do you really believe that premise yourself?
Can you think of another one that would satisfy the original syllogism (Statement X in "if ghosts are real and Statement X is also correct, then mediums' claims are true") that you do believe?
On another note I've just received a copy of Dawkins the God delusion, is it any good?
It sounds like "Demon-Haunted World is probably more applicable to the subjects you're interested in.
Don't think there is any culture in the world that hasn't historically claimed the dead speak to the living.
I've heard of two. One was the Pirahã, but they're famous in anthropology for being so weird & unlike other human cultures in so many ways that a basic description of them sounds like it must be a description of how some not-quite-modern-human relative of modern humans might have lived a couple million years ago, so nothing about them can really be taken as a sign of anything about people in general. And anyway, they do believe in some kinds of spirits, so that's not very far off.
The other was subject of an article titled something like "Shakespeare in the Bush", in which an anthropologist studying some tribe by living among them for a while decided to try to test the idea that Shakespeare is universal by telling them a Shakespeare story and seeing their reactions. When (s)he got to a scene with a ghost talking to a living character, they couldn't imagine why that character would be so scared to find out that a friend/relative (s)he thought was dead was actually alive. When the anthropologist said that character was dead, they'd say "But (s)he's walking & talking, so (s)he's alive". When the anthropologist described what the ghost did & said, they'd say "But you said that character was dead", and around & around they went. They tried imagining everything they could to infuse into the story to get it to make sense, like that it was somebody else in disguise or the living character didn't know what (s)he was seeing because (s)he had been cursed or poisoned. They were convinced the anthropologist was telling the story wrong because the idea of a character being both dead and alive at the same time was beyond merely wrong; it was something they had no concept of to even try to apply the idea of wrongness to.
That kind of thing certainly seems counter to the general trend in by far most human cultures, though.
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