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Peter_de_Blanc comments on Excluding the Supernatural - Less Wrong
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Once, in a LARP, I played Isaac Asimov on a panel which was arguing whether vampires were real. It went something like this (modulo my memory): I asked the audience to define "vampire", and they said that vampires were creatures that lived by drinking blood.
I said that mosquitoes were vampires. So they said that vampires were humanoids who lived by drinking blood.
I said that Masai who drank the blood of their cattle were vampires. So they said that vampires were humanoids who lived by drinking blood, and were burned by sunlight.
I (may have) said that a Masai with xeroderma pigmentosum was a vampire. And so on.
My point was that vampires were by definition not real - or at least, not understandable - because any time we found something real and understandable that met the definition of a vampire, we would change the definition to exclude it.
(Strangely, some mythical creatures, such as vampires and unicorns, seem to be defined in a spiritual way; whereas others, such as mermaids and centaurs, do not. A horse genetically engineered to grow a horn would probably not be thought of as a "real" unicorn; a genenged mermaid probably would be admitted to be a "real" mermaid.)
So according to you, a mosquito that isn't real is a vampire?
His point is that: P(not real | vampire) ~= 1, which is not the same as: "vampire = not real". It's an if-then relationship, not a logical equivalency.
I understand that Phil was not suggesting that all non-real things are vampires. That's why my example was a mosquito that isn't real, rather than, say, a Toyota that isn't real.
But there's nothing particularly special about a mosquito. It's still an incorrect application of modus tollens. We have: If something is a vampire, then it is not real. From this, we can infer (from modus tollens) that if something is real, then it is not a vampire. Thus, if a certain mosquito is real, it is not a vampire. However, there is nothing here that justifies the belief that if a certain mosquito is imaginary, then it is a vampire.
What's special about a mosquito is that it drinks blood.
Phil originally said this:
Note Phil's use of the word "because" here. Phil is claiming that if vampires weren't unreal-by-definition, then the audience would not have changed their definition whenever provided with a real example of a vampire as defined. It follows that the original definition would have been acceptable had it been augmented with the "not-real" requirement, and so this is the claim I was responding to with the unreal mosquito example.
Ah. That makes more sense.