actually hume's arguments against miracles are rather intricate. they are not aimed at producing evidence for the claim that there have not been and will not be any miracles. what he does is to argue that what is taken to be evidence for the existence of miracles is too flimsy to constitute genuine evidence. (and an appeal to authority can have a place in such an argument. if smart people were to detect miracles under strict observation conditions and everyone reported this event in the same way in detail, then we should have have higher confidence in the testimony of these observers. if only ignorant people who are easily fooled to believe that they are experiencing an event of religious significance report miracles, we should not trust their testimony with the same confidence.) hume also tries to explain why there is such an illusion of evidence. if what we have taken to be the best evidence in miracles is actually too weak for us to rationally believe in miracles, then it is not reasonable to believe that there have been any. this does not entail that there have been no miracles. hume was not out to prove that there could not be miracles, he was out to show that the actual historical claims (in christianity and other religions) have been unreasonable. it is another matter, of course, if he has hit his target.