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Precisely! You narrow your axiomatic system, hoping that one of contradictors will fall out. They've eliminated self containing sets from the Naive Set Theory and Russell's paradox vanished. Hopefully.
It is not "what is it", it is "how does it behave". Whatever behaves as a number, is a number.
If it behaves as it had a sign ... And it does. So yes, it has a sign!
Which is what?