July 18, 2011
The logic of science-6: The burden of proof in law
(For previous posts in this series, see here.)
For a long time, religion claimed to reveal eternal truths. No one except true believers seriously says that anymore because science has become the source of reliable knowledge while religion is increasingly seen as being based on evidence-free assertions. So some believers tend to try and devalue the insights science provides by elevating what we can call truth to only those statements that reach the level of mathematical proof, because such a high bar can rarely be attained and thus everything else becomes a matter of opinion. They can then claim that scientific statements and religious statements merely reflect the speaker's opinion, nothing more.
But science uses criteria other than proof for making judgments about truth. In making such judgments, scientists act more like judges in legal cases than mathematicians deriving proofs. For example, in legal proceedings, the usual practice is to follow the legal principle ei incumbit probatio qui dicit, non qui negat, which I am told (not knowing Latin myself) translates as "the burden of proof rests on who asserts, not on who denies", where the assertion is of a positive nature and not a negative one. So if someone is accused of committing a crime, the burden of proof is on the accuser and not the defendant. This principle is more popularly stated in English as that a person is presumed innocent until proven guilty beyond a reasonable doubt.
This principle is considered such a fundamental aspect of a civilized society that it is enshrined in Article 11 of the Universal Declaration of Human Rights which states that: "Everyone charged with a penal offence has the right to be presumed innocent until proved guilty according to law in a public trial at which they have had all the guarantees necessary for their defence." Of course many countries (including the US) routinely violate this principle when it suits them, while still smugly claiming to uphold the basic principles of human rights.
A point to note is that technically the only outcomes in a legal proceeding are "guilty" (i.e., proved beyond a reasonable doubt) and "not guilty" (not proved beyond a reasonable doubt). The defendant is never proven to be innocent, and has no obligation to do so. Indeed the defendant is not even obliged to provide any kind of defense at all. This can of course lead to undesirable situations where the jury can suspect that a defendant is indeed guilty of the crime but feels obliged to bring in a verdict of not guilty if the case has not met the 'proved beyond the reasonable doubt' standard which is why the 'proven innocent' phrasing is not appropriate for not guilty verdicts. But this kind of undesirable outcome is the price we pay for trying to have the fairest possible system, even if it should lead to public outcries of the sort seen following the not guilty verdicts in the O. J Simpson and Casey Anthony murder trials which were mistakenly interpreted by the public as statements that innocence had been proven when all it meant was that the presumption of innocence had not been contradicted.
One could have an alternative system in which a person is presumed guilty until proven innocent, shifting the entire burden of proof onto the defendant. There is nothing logically wrong with such system but in practice it would be unworkable since there are many more people who are innocent of a crime than there are those who are guilty. Furthermore it is often difficult, if not impossible, to prove innocence. For example, if I am asleep alone at home, it would be very difficult for me to prove that I was not robbing a nearby convenient store at that time, which is why the 'presumed innocent until proven guilty' standard seems to be a better one. So there are good reasons for having the burden of proof be on the person who asserts a positive claim and not on the person who denies as the method of arriving at legal verdicts or 'truths'.
Unless one agrees on which of the two frameworks (presumed innocent until proven guilty beyond a reasonable doubt or presumed guilty until proven innocent) to use in making legal judgments, it may be impossible to agree on a verdict. But whatever system one chooses, the basic structure is that there is a default position that is assumed to be true unless shown otherwise, so that proof of only one position is required.
Similar considerations apply in arriving at scientific truths.
Next: The burden of proof in science