A miracle is a violation of the laws of nature; and as a firm and unalterable experience has established these laws, the proof against a miracle, from the very nature of the fact, is as entire as any argument from experience can possibly be imagined.
So if we’ve never seen it happen, then it can’t happen? I once heard a four-year-old express this principle far more succinctly, when she said, “There’s no such thing of that, ’cause I never heard of it.”
But Hume goes further. He notes that there are claims of miracles, but rules that we must not accept them because there weren’t enough witnesses, or the witnesses lived in times when people were ignorant or in places where they are barbaric. How many witnesses are enough? I don’t think he gave a number. Perhaps 500?