Do extraordinary claims require extraordinary evidence? Is there a burden of proof against extraordinary claims? Should we literally assume that something extraordinary doesn’t exist until it is proven to exist?

Many people say that those who claim that bigfoot, ghosts, and gods exist are making “extraordinary claims” and we should reject the existence of these things because we don’t have enough evidence for them.

What does ‘extraordinary’ mean? It refers to claims that conflict with what we think we know about the world. Many claims are extraordinary because they are extreme (likely false) or potentially impossible.

Extreme

Some claims are extraordinary because they are so extreme. Extreme claims are those that should find to be unlikely given our understanding of the world. Such claims can be about rare characteristics or situations that are unlikely to actually describe anything.

An example of a rare characteristic that is unlikely given our understanding of the world is that someone is nine foot tall. This claim is extreme because the tallest person we know about was 8 foot 11 inches and died in 1940. Saying that someone will be born this year in New York City and will grow to be 8 foot 5 inches tall is similarly extreme because people don’t grow to be that tall very often.

An example of a rare situation that is unlikely given our understanding of the world is that there’s a celestial teapot going around the Sun in our solar system. This claim is extreme because all the teapots we know about are on the planet Earth, they exist here because they are made by humans, humans don’t go to outer space very often, and we don’t know how any teapots would end up going around the Sun.

Potentially impossible

What’s potentially impossible is what might not be able to exist given our understanding of the world. What is impossible is what can’t be true. There are different types of impossibility, such as the following:

Physical impossibility – What can’t be true because it would require the laws of nature to be violated. For example, jumping to the Moon is physically impossible. Some people say that “miracles” are physically impossible, but still occur due to divine or supernatural intervention. Metaphysical impossibility – What can’t be true in any reality. What never happens in any possible world. Not even the supernatural could violate what’s metaphysically impossible. For example, finding a world where water isn’t H 2 O is plausibly metaphysically impossible. Logical impossibility – Logical contradictions. What can’t be true because of logical constraints. For example, it’s logically impossible for Socrates to be both mortal and immortal.

The sense of “impossible” that deals with extraordinary claims is “metaphysical impossibility.” To claim that something is true that we know to be metaphysically impossible is absurd, but to claim that something is true that we suspect could be metaphysically impossible (because of our understanding of the world) is extraordinary. Anything we know to be physically impossible is potentially metaphysically impossible—It might be metaphysically impossible for the laws of nature to be violated. Perhaps there are no possible worlds where the laws of nature are violated.

Miracles are extraordinary precisely because miracles are physically impossible by definition. However, miracles might still be metaphysically possible—we don’t know for certain that there’s nothing supernatural or divine. The claim that miracles exist is extraordinary because the assumption that everything happens because of laws of nature has helped make natural science the best source of knowledge about the world we have. For similar reasons claims that ghosts exist is extraordinary (because we don’t know that it’s metaphysically possible to have a mind without a body). Also, the claim that gods exist is extraordinary because it might be metaphysically impossible for them to exist.

Why would it be impossible for gods to exist? It might depend on the definition of gods. Pantheists claim that gods are identical to the universe. We know that the universe exists, and it could seem trivial that a god exists if we define it as the universe. That is not the type of god we find extraordinary in the sense relevant to this discussion. However, the following types of gods are extraordinary:

A god that can have a mind without a body. All minds seem to require bodies. To have a mind without a body would violate our scientific understanding of the world, and it could be metaphysically impossible. A god that is all-knowing (omniscient) or all-powerful (omnipotent). Science has not confirmed that anything has any of these characteristics, and the scientific evidence we have indicates that knowledge and power is always limited from physical constraints. To be omniscient or omnipotent is as extreme as anything can be, and they might be metaphysically impossible. A supernatural god that can violate the laws of nature is extraordinary in the relevant sense because we don’t know for certain that it’s metaphysically possible for anything to do such a thing. If miracles happen, then they are very rare (and extreme). So far there have been no scientifically confirmed miracles, and making predictions based on the assumption that everything will happen in accordance with the laws of nature has been very successful. Natural science is the most successful study of the world and it always works under the assumption that the laws of nature will not be violated. Natural science is successful precisely because it is so good at making predictions.

What about ordinary claims?

The reason that “extraordinary claims” are singled out is because some claims are so ordinary that we might suspect they are true just because of our understanding of the world. Let’s say we know a neighbor lives in a house. To say that the person who lives in the house is less than ten feet tall is an ordinary claim, and we should believe it. It is probably true because we know of no one who has ever lived taller than nine feet. It is so common for people to be less than ten feet that it is unlikely for such a prediction to ever fail. Such a prediction could very well have a perfect track record for the entirety of human history (and the future of human kind).

Other ordinary claims have a decent probability of being true. For example, to predict that a neighbor is less than seven feet tall is still a decent prediction, even though some people are taller than that.

Ordinary claims are known to be metaphysically possible. We know it is metaphysically possible for a person to be under ten feet tall.

Of course, there are gray areas that are neither ordinary nor extraordinary. Scientists hypothesize about laws of nature that might not exist. Such laws of nature might actually encourage scientists to make predictions that would violate the actual laws of nature (and be metaphysically impossible as a result). However, scientists should never hypothesize that a law of nature exists that we know contradicts the actual laws of nature. A hypothesis that repeatedly fails to make certain predictions will be rejected precisely because we will find out that it violates the actual laws of nature that actually exist.

There is a burden of proof against extraordinary claims

People know something about the world and there are many scientific observations that are relevant to the claims we make. If someone makes an extraordinary claim (a claim that contradicts what we think we know about the world), then we should reject the claim until we are given a good reason to change our mind. The burden of proof is the requirement that those who make extraordinary claims have in order to rationally change our minds.

The existence of ghosts and tiny faeries are extraordinary. The scientific view of the world states that minds can’t exist without bodies, and that bigger brains are required to have the intelligence of a human being. Ghosts have minds without bodies; and faeries are very small, but have the intelligence of a human being. These entities literally violate what we think we know about the world. We should be hesitant to believe in ghosts and faeries. We should require at least some evidence that these entities exist before deciding they exist.

We can certainly imagine having a good reason to change our mind and think ghosts or faeries exist. Perhaps we can literally find a tiny faerie that talks and interacts with us over a period of years. In that case the faerie’s existence could be the best explanation for our experiences (as opposed to a hallucination or dream).

Once we find out that ghosts or faeries exist, that will require us to revise our understanding of the world. We might find out that minds don’t require bodies, or that a tiny brain can be capable of having a human level of intelligence. Once we find out that ghosts or faeries exist, such a claim will no longer be extraordinary for those of us who find out that they exist. If anyone finds out that ghosts or faeries exist, then such people will not require a burden of proof that they exist because that debate will be over. Of course, anyone who believes in ghosts of faeries should stay open to the possibility that they are wrong, and the debate can be resumed at some point.

Do extraordinary claims require extraordinary evidence?

Extraordinary claims don’t require some strange type of evidence that is so different from an ordinary type of evidence. Extraordinary claims can be proven to be true in much the same way as any other type of claim. In fact, extraordinary claims are not totally different from other types of claims. They are far fetched, but all claims are part of a single continuum that ranges from “far fetched” to “obviously true.”

Obviously true claims require no evidence, such as the claim that the person who lives in a house is under ten feet tall. The reason is simply because what we know about the world already proved it to be true. The evidence has already been attained, and no further evidence is required.

Extraordinary claims require extraordinary evidence in the sense that they require more evidence than usual before they are properly proven to be true. Claims that violate what we think we know about the world require more evidence because certain other claims we think were proven to be true are incompatible with them. For example, we think we know that minds require bodies, and the existence of ghosts violates that. We already have evidence that minds require bodies. In fact, we think we know certain types of bodies can have minds (such as that of a living mammal) and others can’t have mind (such as that of a plant). So, we think we know that there are only certain types of bodies that can have minds.

Before we decide that ghosts exist, we should require better evidence that ghosts exist than the evidence we have that minds require bodies. We should not reject one well-justified belief (such as the belief that minds require bodies) just because we want to accept an extraordinary claim that contradicts the belief (such as the belief that ghosts exist). However, it is theoretically possible to have better evidence that ghosts exist than that minds require bodies. If we find a way to interact and talk with ghosts, perhaps we will eventually know that ghosts exist.

We do have some evidence that ghosts exist. People say they have seen ghosts, talked to them, or been touched by them. People have had observations, and they speculated that ghosts are an explanation of those observations. The problem is that there can be alternative explanations of our ghost experiences. One common alternative explanation is “misidentification.” We can simply think a ghost caused an observation when something else actually caused it. Hallucinations and dreams are also possible alternative explanations when more ordinary explanations are ruled out.

Should we literally assume that something extraordinary doesn’t exist until it is proven to exist? Yes, we should assume a claim to be false when it contradicts what we think we know to be true. We should assume that a person didn’t jump to the Moon, and we should assume that the snake oil doesn’t actually cure all ills precisely because it would defy reasonable expectation.

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