I haven't read the other posts before responding to your questions:

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1. Is there a bear in the room you are in now?

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No.

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2. Is there a flea in the room you are in now?

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Not sure.

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3. Are you sure?

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#1: Yes. #2: No (yes, I'm sure I'm not sure).

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4. Is the lack of evidence proof that something is not there?

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No, but it's a good place to start. Until there is evidence, or inductive reasons to believe something is there, then it's probably not.