Hume on Miracles
Hume defines a miracle as: "a transgression of a law of nature by a particular volition of the Deity, or by the interposition of some invisible agent." In other words, a miracle is a specific act by God (or an angel, etc.) that breaks one of the laws of nature that God created. [If you think that a miracle is any act of God, such as, perhaps, the daily operation of the laws of nature, then you have a different understanding of a miracle than Hume's.]
Hume argues that we should not believe that miracles, so defined, ever happen. They could, but it is always more reasonable or wise to believe of any particular alleged miracle that it did not happen, that someone has made a mistake or has lied.
"A wise man proportions his belief to the evidence." The main evidence we have for any belief is our own experience, but we should be somewhat skeptical about this, since it might not represent the whole truth. When we base our beliefs on other people's testimony about their experiences we should be even more wary. Not all witnesses are reliable.
When there is disagreement in our experience or the testimony presented to us, we should weigh the different sides against each other. In favor of the laws of nature having no exceptions is the whole weight of science and (almost) universal human experience. In favor of their having been an exception at some time to one or more of these laws is the testimony of a few people who lived far away and long ago. It's pretty obvious where smart people will put their faith. [But, from Hume's point of view, why should we believe in laws of nature in the first place? The evidence for them is all inductive, and inductive reasoning is not rational, according to Hume. The wise people he has in mind seem to be people of common sense or ordinary belief, not strictly rational people.]
Why then would anyone ever claim to have seen a miracle? Perhaps they have been overcome by religious enthusiasm and suffered a kind of hallucination. Perhaps they think they can bring about conversions by telling a few white lies. Or perhaps they just want attention, respect, or a chance to make a few bucks selling their story. Whatever the reasons, we know that not all stories of miracles are genuine. Some have been proved false. [But in trying to judge such a story, how is evidence to be weighed? What on earth is the probability of God's existing and performing certain acts? Hume seems to think it is very small, but in fact it just seems impossible to say.]
Indeed, not all stories of miracles can be true. There are such stories claiming to prove almost every religion there has ever been, and they cannot all be true. So why believe one set of stories rather than another? That would be arbitrary. The wise person will instead believe none of them. The Bible is not necessarily more reliable than any other religious book, and they cannot all be true since what they say is often quite incompatible.
Religious belief requires a kind of faith that is so contrary to reason and experience that it is a miracle that it exists at all, Hume quips.
[There might seem to be a problem here, since Hume thinks it is not rational to believe in laws of nature in the first place. But, as he defines them, miracles require there to be laws of nature. Without laws of nature there cannot be violations of laws of nature. So people who believe in miracles--in Hume's sense--must believe in laws of nature. That is, they believe that there are certain ways that events in the world must go. (If they only believe that there are ways that things tend to go then they do not believe in laws of nature.) But then they also believe that the omnipotent, omniscient, and perfectly good author of these laws breaks them from time to time. If these laws bind creatures but not God, then that seems possible. Now, why do they believe this? They believe in laws of nature, presumably, on the basis of experience, of empirical evidence. They believe in violations of the laws on the same basis. If books like the Bible are good empirical evidence, then we should not, it seems, believe that it is a law of nature that seas cannot part, for instance. (Unless, as I said, laws of nature are not taken to bind or limit God and his invisible agents.) And if that is not a law of nature, then the parting of the Red Sea reported in the Bible is not a miracle. Alternatively, if we do not base our understanding of the laws of nature on sources such as the Bible, then we can believe in such miracles, but it will not be empirical of us to do so. Hume does not say this, but perhaps what he is getting at is the idea that science and religion do not mix. If we treat them as the same kind of thing, treating books like the Bible as evidence of historical and scientific fact, then we destroy the possibility of belief in miracles. If we regard religious scriptures as books of a different kind, then we cannot claim that faith in them is rational, whatever other virtues it might have.]