Turning to civil law, there is no simple way to explain burden of proof. Very roughly, the plaintiff (the one who brings the suit) has an initial burden to establish a prima facie case—that is, a case that is strong enough that it needs to be rebutted—on behalf of his or her complaint. The burden then shifts to the respondent (the one against whom the suit is being brought) to answer these claims. The burden may then shift back and forth depending on the nature of the procedure. Provided that both sides have met their legally required burdens of proof, the case is then decided on the basis of the preponderance of evidence; that is, the judge or jury decides which side has made the stronger case.
Burden of proof is primarily a legal notion, but it is sometimes used, often loosely, outside the law. The notion of burden of proof is needed within the law because law cases are adversarial and the court has to come to a decision. Outside the law, people have a general burden to have good reasons for what they say. That is the second part of Grice’s rule of Quality. More specifically, people have a burden to be able to present some reasons when they make accusations or state- ments that run counter to common opinion.
The important thing to see is that you cannot establish the truth of some- thing through an appeal to the burden of proof. The following argument is perfectly weird:
There is life in other parts of the universe, because you can’t prove otherwise.
Of course, no one can prove that there is not life elsewhere in the uni- verse, but this has no tendency to show that there is. Attempts to prove the truth of something through appeals to burden of proof—often called arguments from ignorance—are another example of a fallacy of relevance.
Nonetheless, the importance of burden of proof in the law does give force to another kind of argument. In a criminal case, the following argument would be perfectly fine:
The defendant ought to be found not guilty, because the prosecution has not proven beyond a reasonable doubt that she is guilty.