The term "supernatural" is frequently used in discussions related to skepticism. I am trying to establish the category that people refer to with this term.
All uses of this term appear to imply a separation of concepts and events into two disjoint categories: "natural" and "supernatural". Some examples of things typically classified into "supernatural": God, ghosts, telepathy, telekinesis, aura. Things typically classified as "natural": animals, rocks, talking, earthquake, body temperature.
I will try to follow the advice given in Similarity Clusters and try to establish some verbal hints as to what causes a concept to be classified into either similarity cluster.
One idea I had is the following: anything we expect to be able to experience, if the necessary prerequisites are met, is "natural"; anything we expect to fail to experience even if we try hard is "supernatural". This seems to work quite well on the concepts mentioned above. This works for unlikely events too: a plane crash is not "supernatural" because if I'm at the right place and the right time then I expect to be able to experience it.
It's still a bit weak for exceedingly unlikely events. For example, proton decay has never been witnessed, and we don't know if it can even occur. But "proton decay" is not classified as "supernatural"; rather as a "hypothesis". Telepathy, however, might for all we know be as rare as proton decay (thus being exceedingly hard to confirm experimentally), and yet it's classified into "supernatural". Something is missing from this verbal hint.
Approaching this from a different perspective, it appears that one can classify "supernatural" as having the property of being "outside of the universe". On further thought, however, this isn't helpful at all: the latter is not so much a verbal hint as a label in itself.
If taken literally, one might argue that all supernatural things therefore don't exist. They are said to be outside the universe, but we can only experience things within the universe, because anything we can experience must be part of the universe, and thus "inside" it. This is quite useless, however, in my opinion: as used by actual people, the category "supernatural" isn't intended to preclude existence. So this doesn't work.
Could it be that the category "supernatural" is actually completely useless, by offering so little information about the things that belong to it that knowing that something is classified as "supernatural" doesn't tell us very much at all?
Thinking about this led me to the idea that perhaps "supernatural" simply means "something that science has shown false or doesn't accept as a valid theory". That is certainly a property I infer about P when told that P belongs to "supernatural".
This is still quite unsatisfactory. It can't be the only property. People explain away God's undetectability by being "supernatural", intending it as a convincing argument - but even those who do things like this wouldn't claim that "not a valid theory" is an argument in favour of God. They must mean something else.