Your definition is a contradiction. Given X is an ordered-set element with ordinal number n iff not-X is an ordered-set element with ordinal number m>n, therefore if not-X is an ordered-set element with ordinal number m, then not-not-X = X must be an ordered-set element with ordinal number n>m, therefore n>m>n, therefore n != n.
It's also a mis-paraphrasation of Hume, who describes a sound, although rough, probabilistic heuristic for guessing whether an alleged event transpired, not a definition of any kind.
See the relevant SEP article.
Addendum edit:
Hume does define a miracle earlier in Enquiry: "a violation of the laws of nature." By "laws of nature" he seems to mean the collected knowledge of events which experience has told us are common. I have some problems with that apparent meaning, but that's not relevant to this post.