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.
But the same exchange might have occurred with something entirely real. We are not in the habit of giving fully adequate definitions, so it is often possible to find counterexamples to the definitions we give, which might prompt the other person to add to the definition to exclude the counterexample. For example:
A: What is a dog?
B: A dog is a four-footed animal that is a popular pet.
A: So a cat is a dog.
B: Dogs bark.
A: So if I teach a cat to bark, it will become a dog.
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.
But the same exchange might have occurred with something entirely real. We are not in the habit of giving fully adequate definitions, so it is often possible to find counterexamples to the definitions we give, which might prompt the other person to add to the definition to exclude the counterexample. For example:
A: What is a dog?
B: A dog is a four-footed animal that is a popular pet.
A: So a cat is a dog.
B: Dogs bark.
A: So if I teach a cat to bark, it will become a dog.
etc.