Since when is asserting a negative a "positive claim"? That's ridiculous.
If a person is found dead with no signs of murder at all, but somebody believes I murdered that person and I positively assert that I didn't murder them, it is not up to me to prove that the person wasn't murdered and that I didn't do it. It's up to them to prove that the person was murdered and then that I did it.
Same goes if I positively assert that Uranus isn't populated by a cannibalistic race bearing an uncanny resemblance to Winnie The Pooh and the burden of proof would lie with any person claiming it is.
If a person is found dead with no signs of murder at all, but somebody believes I murdered that person and I positively assert that I didn't murder them, it is not up to me to prove that the person wasn't murdered and that I didn't do it. It's up to them to prove that the person was murdered and then that I did it.
Same goes if I positively assert that Uranus isn't populated by a cannibalistic race bearing an uncanny resemblance to Winnie The Pooh and the burden of proof would lie with any person claiming it is.