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May 11 2006

Burden of proof-3: The role of negative evidence

In my previous post, I suggested that in science, the burden of proof lies with the proponent for the existence of some thing. The default assumption is non-existence. So if you propose the existence of something like electromagnetic radiation or neutrinos or N-rays, then you have to provide some positive evidence that it exists of a kind that others can try to replicate.

But not all assertions, even in science, need meet that positive evidence standard. Sometimes negative evidence, what you don’t see, is important too. Negative evidence is best illustrated by the famous Sherlock Holmes story Silver Blaze, in which the following encounter occurs:

Gregory [Scotland Yard detective]: “Is there any other point to which you would wish to draw my attention?”
Holmes: “To the curious incident of the dog in the night-time.”
Gregory: “The dog did nothing in the night-time.”
Holmes: “That was the curious incident.”

There are times when the absence of evidence can be suggestive. This is true with the postulation of universal laws. The substance of such laws (such as that the total energy is conserved) is that they hold in every single instance. But we cannot possibly examine every possibility. The reason that we believe these types of laws to hold is because of negative evidence, what we do not see. If someone postulates the existence of a universal law, the absence of evidence that contradicts it is taken as evidence in support of the law. There is a rule of thumb that scientists use that if something can happen, it will happen. So if we do not see something happening, that suggests that there is a law that prevents it. This is how laws such as baryon and lepton number conservation originated.

Making inferences from absence is different from proving a negative about the existence of something, be it N-rays or god. You can never prove that an entity doesn’t exist. So at least at the beginning, it is incumbent on the person who argues for the existence of something to provide at least some evidence in support of it. The case for the existence of entities (like neutrinos or X-rays or god) requires positive evidence. Once that has been done beyond some standard of reasonable doubt, then the burden can shift to those who argue for non-existence, to show why this evidence is not credible.

This rule about evidence was not followed in the run up to the attack on Iraq. The Bush administration simply asserted that Iraq had weapons of mass destruction without providing credible evidence of it. They then (aided by a compliant media) managed to frame the debate so that the burden of proof shifted to those who did not believe the weapons existed. Even after the invasion, when the weapons did not turn up, Donald Rumsfeld famously said “There’s another way to phrase that and that is that the absence of evidence is not the evidence of absence. It is basically saying the same thing in a different way. Simply because you do not have evidence that something does exist does not mean that you have evidence that it doesn’t exist.” But he was wrong. When you are asserting the existence of an entity, if you have not provided any evidence that they do exist, then the absence of evidence is evidence of absence.

It is analogous to criminal trials. People are presumed innocent until proven guilty, and the onus is on the prosecution to first provide some positive evidence. Once that is done, the accused usually has to counter it in some way to avoid the risk that the jury will find the evidence sufficiently plausible to find the accused guilty.

So the question boils down to whether believers in a god have provided prima facie evidence in support of their thesis, sufficient to shift the burden to those who do not believe in god to show why this evidence is not convincing. Personal testimony by itself is usually not sufficient in courts, unless it is corroborated by physical evidence or direct personal observation by other credible sources who have observed the same phenomenon.

One of the common forms of evidence that is suggested is that since many, many people believe in the existence of god, that should count as evidence. My feeling is that that is not sufficient. After all, there have been universal beliefs that have subsequently been shown to be wrong, such as that the Earth was located at the center of the universe.

Has the evidence for god met the standard that we would accept in science or in a court of law? I personally just don’t see that it has but that is a judgment that each person must make. Of course, people can choose to not require that the evidence for god meet the same standard as for science or law, and if that is the case, then that pretty much ends the discussion. But at least we can all agree as to why we disagree.

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