Skeptics are fond of quoting a certain passage by David Hume relating to miracles, written in his book An Enquiry Concerning Human Understanding. The passage is as follows:
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. Why is it more than probable, that all men must die; that lead cannot, of itself, remain suspended in the air; that fire consumes wood, and is extinguished by water; unless it be, that these events are found agreeable to the laws of nature, and there is required a violation of these laws, or in other words, a miracle to prevent them? Nothing is esteemed a miracle, if it ever happen in the common course of nature. It is no miracle that a man, seemingly in good health, should die on a sudden: because such a kind of death, though more unusual than any other, has yet been frequently observed to happen. But it is a miracle, that a dead man should come to life; because that has never been observed in any age or country. There must, therefore, be a uniform experience against every miraculous event, otherwise the event would not merit that appellation. And as a uniform experience amounts to a proof, there is here a direct and full proof, from the nature of the fact, against the existence of any miracle; nor can such a proof be destroyed, or the miracle rendered credible, but by an opposite proof, which is superior. The plain consequence is (and it is a general maxim worthy of our attention), ‘that no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavors to establish; and even in that case there is a mutual destruction of arguments, and the superior only gives us an assurance suitable to that degree of force, which remains, after deducting the inferior.
The reasoning in this passage involves a number of fallacies. The problem starts at the very beginning where Hume defines a miracle as something that “is a violation of the laws of nature.” If you define a law of nature as something that always happens (a typical definition), and then define a miracle as something that violates such a law (something that always happens), then you are starting out by defining a miracle as something that can never happen. That is not fair if you considering whether miracles might happen, because it is a way of begging the question. It's rather like starting out an essay considering the possibility of ESP by defining ESP as an “impossible type of cognition.”
As it happens, some common types of things thought of as miraculous do not clearly involve violations of laws of nature. If some miracle worker or mystic or medium were to levitate a table or some other object (something that has actually been reported quite a few times), we need not at all think of this as a violation of the law of gravitation. Gravitation is the weakest force of nature, and even a very small force is able to overcome it (for example, you can cause an object to rise through air jets, magnetism, and so forth). So if we see something levitating, we need merely think “a small unknown force has been applied underneath the object, causing it to rise” rather than extravagantly thinking “the law of gravitation has been locally violated.” Similarly, a case of something apparently appearing out of nowhere need not be thought of as a violation of the law of the conservation of mass and energy, as there are all kinds of unknown ways in which local nearby matter might be transformed in a way that does not violate such a law. Similarly, cases of remarkable healing or resuscitation after apparent death do not violate any known laws of physics or chemistry, which say nothing about biology. Nor do they violate any laws of biology, which has no such laws as “cancer never reverses” or “people always die as soon as their vital signs stop.”
There is a better definition of miracle, which is simply something extraordinary and marvelous that is not explicable by natural or scientific explanations. Once we use that better definition, it's a great blow to Hume's reasoning. Another problem with Hume's reasoning is the naivete involved in its consideration of laws of nature.
Here are the kind of assumptions that Hume seems to make:
- Nature always behaves in fixed or predictable ways, without throwing us much in the way of surprises.
- Humans can figure out from their experience what these fixed tendencies are.
- Once humans have figured out these tendencies, they are then entitled to exclude any report of experiences that differ from these tendencies that have been observed in the past.
There are all kinds of reasons why such assumptions are invalid. For one thing, very often nature often throws us surprises, such as earthquakes, volcanic eruptions, meteors, and many types of anomalous phenomena. Secondly, humans are generally not smart enough or knowledgeable enough or experienced enough to be declaring that some particular tendency is an inviolable law of nature, based on our small experience which occurred in only the tiniest “zillionth” sliver of the universe's history, and occurred while living on only a “zillionth” iota of the universe's territory. Third, from the mere fact that we always have seen X happening, we are not entitled to conclude that X must always happen (unless we have solid knowledge of reasons why X must always happen). And from the fact that we have never previously seen X, we are not entitled to conclude that we cannot now start observing X.
For example, if you were an American colonist of Hume's time (around 1770), you might have deduced that it was a law of nature that big earthquakes don't occur anywhere near the American colonies, based on the “firm and unalterable experience” that no such earthquakes had been observed. But such a law would have been invalid, and very large earthquakes did occur in Missouri in 1811 (the New Madrid earthquakes). Similarly if you were a Roman living in Pompeii in 78 AD, you might have assumed it was a law of nature that “mountain tops never explode,” but you would have been soon surprised when just such a thing happened in the eruption that buried Pompeii.
Such examples show how mistaken Hume was when he wrote this line from the passage cited above:
And as a uniform experience amounts to a proof, there is here a direct and full proof, from the nature of the fact, against the existence of any miracle; nor can such a proof be destroyed, or the miracle rendered credible, but by an opposite proof, which is superior.
No, a uniform experience does not amount to a proof. Even if humans have always experienced X, this does not prove that something different from X may some day occur. And even if humans have never experienced X, it is no proof that they will not in the future experience X.
What Hume seems to suggest is a kind of reasoning which we might visualize by imagining a scale. On the left side is human experience suggesting that X does not happen. On the right side is some human experience suggesting that X did happen. Unless the right side outweighs the left side, according to Hume, we should not believe that X happens. But this makes no sense. If people in 100 cities are not blown up by asteroids, and they all conclude that “asteroids don't blow up cities,” we do not need 101 cities to be blown up by asteroids to show that an asteroid can blow up a city. A case of a single city being blown up by an asteroid is sufficient to show such a thing. Before September 11, 2001, it was generally believed that fires can't cause skyscrapers to fall, based on 1000 skyscraper fires. But we didn't need 1001 skyscrapers falling during fires to show that such a thing can happen; we only needed one of them like the two that fell on September 11, 2001.
What's ironic is that in many other places in his writings, Hume was very skeptical about human knowledge and human abilities. He often tended to think of human beings as error-prone stumblers and bumblers, and he famously claimed that reason is the slave of the passions. But when it came to considering man's ability to proclaim all-encompassing laws of nature from man's limited experience, Hume “checked at the door” his usual skepticism, and spoke in a very naive way as if such laws could be proclaimed with complete confidence. It's the same deal for many modern skeptics: they bring out unbounded skepticism for people reporting the anomalous, but zero skepticism towards those who are dogmatically declaring that such things can't happen.
Another commonly quoted statement by Hume is the one immediately following the paragraph quoted above. It goes like this:
When anyone tells me, that he saw a dead man restored to life, I immediately consider with myself, whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened. I weigh the one miracle against the other; and according to the superiority, which I discover, I pronounce my decision, and always reject the greater miracle. If the falsehood of his testimony would be more miraculous, than the event which he relates; then, and not till then, can he pretend to command my belief or opinion.
This is often used as some justification for denying evidence of the paranormal. But the problem is that claims of the paranormal usually don't rest on a single person's testimony. The better claims of the paranormal are supported either by the testimony of many witnesses, or large amounts of photographic evidence or laboratory evidence.
A photo I took yesterday, described here.
In some cases there are two of these three things to support the claim. For example, evidence for ESP includes testimony by countless observers and also lots of good laboratory evidence; and evidence for UFO's includes a wealth of witness testimony and photographic testimony. It's therefore fallacious to speak as it's all a question of whether we should believe one person's word.
Ironically, the very thing that Hume used as an example of testimony that can't possibly be true has actually been proven true. See here for quite a few cases of people who revived from death quite a while after they had been declared dead, with no vital signs. This is another example of why it's wrong to assume that some thing can't happen merely because it wasn't previously observed.