Burden of Proof
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In law, the burden of proof refers to the obligation of one participant in an action to prove allegations. In less formal contexts, the phrase is often used to describe the different degrees of evidence required for a hypothetical dispassionate and rational observer to believe one's claims. For example, in scientific debate, the less a priori reasonable a claim is, the greater the amount and quality of evidence that will typically need to be presented before one's claim is believed.
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[edit] Discussion
[edit] In Law
In common law jurisdictions such as England and the United States, there is a principle of criminal law that one is "innocent until proven guilty." In plainer language, this means that if the prosecution is unable to "prove" the defendant's guilt (for example, perhaps there are two equally plausible theories of the crime, and in only one of them is the defendant guilty), the defendant must be found innocent. Similarly, if the prosecution can prove (via forensic evidence) that the victim was murdered by a shot from a particular gun, but cannot prove that the defendant fired the gun, the defendant must be found innocent.
Burden of proof, and specifically the standard of proof that must be met, varies with jurisdiction and with legal context. The two most important standards are the so-called "preponderance of the evidence" standard and the "beyond a reasonable doubt" standard. "Beyond a reasonable doubt" is usually found in criminal trials, where it is the responsibility of the prosecution to produce evidence of sufficient credibility and quality that there is no doubt left in the mind of a "reasonable person". In civil trials, the usual standard is "the preponderance of the evidence," which simply means that in the mind of the self-same "reasonable person," the claim is regarded as more likely to be true than it is to be false. This is obviously a much weaker standard of proof; the defendant in a criminal trial can achieve acquittal by suggesting an unlikely but possible theory of what really happened, but the defendant in a civil trial may still be charged with substantial damages in a related trial. This is not a purely hypothetical circumstance, as it describes the actual events of O.J. Simpson's trials for the death of his wife Nicole.
[edit] Science and Philosophy: Falsification
There is one important set of circumstances under which it is clear who has the burden of proof. When one party makes a general claim, and the opposing party claims the existence of a counterexample, then the burden of proof lies on the person claiming the counterexample.
Example:
- A: Pigs can fly.
- B: No they can't.
- A: Oh yes? Prove it. Show me that no pig anywhere in the world right now is flying, and that none of them ever have or ever will.
- B: No, the burden of proof is on you. Show me one flying pig.
In this instance, B is correct. This example shows a pragmatic reason for the rule; B would have to do an impossible amount of work to prove his case even if B is right; whereas A has a much lighter task --- if A is right. Indeed, if A has any basis for making his assertions about flying pigs, then presumably he has access to evidence for this and his task is simple indeed.
This relates to the idea of falsification in the philosophy of science. If we consider, for example, the theory of gravity, which makes statements about the physics of all bodies in all places and at all times, we cannot of course verify this by observing all bodies at all places and times. However, we cannot on that basis reject the theory of gravity --- we need it to do physics and engineering and so forth. For the same reason, we can't even give parity to the notion that it's correct and the notion that it isn't. Instead, we have to accept the theory as correct until someone produces evidence falsifying it.
Trying to reverse the burden of proof is a common fallacy practised by people who would otherwise have a hopeless case, e.g. a 9/11 Truther demanding evidence that the events of 9/11 weren't faked by evil Jews, or a creationist complaining that evolution hasn't been validated until paleontologists have looked at every organism in the fossil record to check that it fits the theory. By this means they mean to argue at least for parity between well-established theories and their own flimsy hypotheses, on the grounds that neither can be proved if the burden of proof is shifted in this way.
This manoeuvre falls under the category of special pleading. For the same people who juggle the burden of proof when it comes to their pet conspiracy theory or favorite pseudoscience do not also demand parity between the correctness and incorrectness of the theory of gravity; nor between the proposition that pigs can fly and that they can't; nor between the proposition that they are guilty of murder and that they aren't. Such evasions are only ever used to protect a favored but flimsy hypothesis in competition with a well-supported theory.
[edit] Science and Philosophy: Extraordinary Claims
There is rarely a formal obligation outside of legal circles to present any kind of evidence, but participants in a discussion are likely to use a failure to present evidence as a guide to the credibility of any claims made. Because science as it is generally understood has a well-established track record of being able to successfully explain and predict the world, the currently accepted body of scientific knowledge is generally considered to be true by practicing scientists and philosophers. This has led to a certain degree of epistemological conservatism among these people, where a claim that violates normal science is likely to be met, initially, with disbelief.
The degree of disbelief is related to the degree that the claim violates established norms. A claim, for example, of the discovery of a new species of butterfly would not be particularly controversial, since there's no strong experimental or theoretical backing to the claim that we know all of the existing butterfly species. A claim to have discovered a fire-breathing dragon would be greeted with much more skepticism, because, first, there are many fewer places left in the world where a large carnivore such as a dragon might live unsuspected; second, dragons (as hexapedal vertebrates) violate many of the established norms of biological taxonomy; and third, there is good theoretical reason to believe that a fire-breathing animal is impossible as it would burn its own lungs. A claim that violates fundamental laws of science such as the second law of thermodynamics, the theory of evolution, or the law of conservation of energy would be met with still greater skepticism, as these laws are extremely well-established both experimentally and theoretically.
This inherent disbelief establishes an implicit "burden of proof" on the proponent of a new fact, interpretation, or theory, and the less reasonable the theory is, the greater the implicit burden upon the proponent. This is often presented as the maxim "Extraordinary Claims Require Extraordinary Evidence", where the proponent of a so-called "extraordinary" theory is required to establish an extraordinary amount of evidence in favor of his claim before he can hope to have it accepted.
[edit] An Example
A good, perhaps classic, example of the informal "burden of proof" in cinema comes from the cult-classic movie Plan 9 From Outer Space. At the end of this movie, the narrator states "My friends, you have seen this incident, based on sworn testimony. Can you prove that it didn't happen?" Since the "incident" in question involves the extraordinary notion of space-travelling grave-robbers reanimating dead bodies, it is apparent that the "burden of proof" should be on the filmmaker to establish that it did happen, and not the other way around -- and thus that the burden of proof is being misassigned.
