Shifting the burden of proof
Shifting the burden of proof is a kind of logical fallacy in argumentation whereby the person who would ordinarily have the burden of proof in an argument attempts to switch that burden to the other person, e.g.:
- If you don't think that the Invisible pink unicorn exists, then prove it!
In an argument, the burden of proof is on the person making an assertion. That is, if a person says that the moon is made of cheese, then it is up to that person to support this assertion. Demanding that the other party demonstrate that the moon is not made of cheese would constitute shifting the burden of proof.
It can sometimes be tricky to determine who holds the burden of proof for a given assertion. In the example above, if the other person in the argument said that we know that the moon is not made of cheese because a shuttle mission collected moon samples and they were made of rock, then that person would now have the burden of proving that the shuttle can reach the moon.
Well-established scientific theories
Often, in debates over well-established scientific theories, the person arguing against the mainstream view will say that it is not up to him to disprove the theory, but that it is scientists' job to demonstrate it.
This is true, in a sense: when a new scientific hypothesis is introduced, its proponents have the onus of demonstrating it. The rest of the scientific establishment has no obligation to disprove the new hypothesis. However, a hypothesis can only rise to the rank of theory by being repeatedly tested, and by accumulating evidence in its favor. This evidence must now be taken into account by the theory's critics.
Thus, if person A says that relativity is unproven, and person B asks A for evidence, this may be seen as shifting the burden of proof, but B is really asking A to support the positive assertion that the mass of evidence for relativity is not conclusive.
In online debates, when a person challenges a well-established scientific theory, it is almost invariably the case that that person does not know or does not understand the evidence for the theory.