Usually, the argument that a miracle occurs is that someone claims to have seen one. And this is where Hume's essay comes in, as he discusses how one should evaluate the testimony for a miracle.
As for the cause of a miracle, that is, as you say, a different question. Usually, though, the testimony for the miracle also includes some claim about who or what caused it. One could, theoretically, accept that the miracle did occur, but not believe the testimony regarding who or what caused the miracle. The thing is, if the testimony is unreliable (and it would be, if one is rejecting the claim of who or what is responsible), why believe the testimony that the miracle occurred at all? But, as you note, it is a separate question.
As for the cause of a miracle, that is, as you say, a different question. Usually, though, the testimony for the miracle also includes some claim about who or what caused it. One could, theoretically, accept that the miracle did occur, but not believe the testimony regarding who or what caused the miracle. The thing is, if the testimony is unreliable (and it would be, if one is rejecting the claim of who or what is responsible), why believe the testimony that the miracle occurred at all? But, as you note, it is a separate question.
"A wise man ... proportions his belief to the evidence."
— David Hume, An Enquiry Concerning Human Understanding, Section X, Part I.
