I think Luna is tuned into the difference between Harry’s epistemology and Hermione’s, and so thinks the question is worth asking in the one case and not the other; alternatively, she is tuned into how they respond to questions of that sort, and Hermione invites them whereas Harry does not.
One is a response to a statement which is positive (not ‘it exists’ but ’it is (there)), while the other is a statement which is negative (it does not exist, or specifically:
“There is no Platform Nine and One-Half,” the unicorn replied.
“How do you know?” Luna asked.
).
For completeness, here is the other one:
“It’s in the Mirror of Atlantis,” Harry said.
“We need Gillyweed,” said Luna, “Lots of Gillyweed.”
“The Mirror of Atlantis is located at the end of the third-floor corridor on the right-hand side,” said Harry.
“What are you waiting for?” said Luna.
“You’re not going to ask me how I know these things?” said Harry.
There are other differences between surrounding contexts, but that difference really pops out—they’re different responses to different types of claims. (Also, arguably one is magical, one is mundane/numeric.)
But, chapter 1:
Is it only some knowledge that cannot be gained from nothing? Such as that a thing does not exist?
I think Luna is tuned into the difference between Harry’s epistemology and Hermione’s, and so thinks the question is worth asking in the one case and not the other; alternatively, she is tuned into how they respond to questions of that sort, and Hermione invites them whereas Harry does not.
“Why would I do that?” and “You think like a muggle” sound like she thinks Harry is making an epistemological error.
If she were as tuned in as you say, she should see that Harry asks because he doesn’t see how tuned in Luna is.
One is a response to a statement which is positive (not ‘it exists’ but ’it is (there)), while the other is a statement which is negative (it does not exist, or specifically:
).
For completeness, here is the other one:
There are other differences between surrounding contexts, but that difference really pops out—they’re different responses to different types of claims. (Also, arguably one is magical, one is mundane/numeric.)