Alan4discussion writes It is indeed jay29 who is "begging the question", by asserting the existence of an undefined "supernatural" and then asking others to disprove this undefined and unevidenced assertion.

No, you are confused here and commit a category error. A fallacy of begging the question pertains to arguments not to (single) asserted propositions or questions; hence there was no fallacy from me.

Any philosopher's definition of the "supernatural" is likely to be based on" imaginary concepts" in magical stories from mythology.

I have no idea what you mean here. My Penguin dictionary of philosophy defines "supernatural" as: "supernatural adj., n. Supernatural beings exist above or beyond nature, where "nature" is to be understood in a wide sense to take in all of space and time and everything existing within that framework, i.e. the whole of the physical universe. It is especially in the context of religious belief that the concept of the supernatural has been used...supernatural beings run no risk of having their existence disclosed by scientific or everyday observation." This definition seems to correspond to the manner in which the term "supernatural" is used in philosophical or, indeed, everyday discourse. I do not consider that arguments for a supernatural realm are, for the most part, best presented as "stand- alone" arguments: rather someone arguing for such a supernatural realm might argue for the existence of a deity from which (with the aid of auxiliary premises) one might infer the existence of a supernatural realm.