The concept of a "burden of proof" is important in debates—whoever has a burden of proof is obligated to "prove" their claims in some fashion. If someone doesn't have a burden of proof, then their job is much easier: all that is required is to either accept the claims or point out where they are inadequately supported.

It is therefore no surprise that many debates, including those between atheists and theists, involve secondary discussions over who has the burden of proof and why. When people are unable to reach some sort of agreement on that issue, it can be very difficult for the rest of the debate to accomplish much. Therefore, it is often a good idea to try to define in advance who has the burden of proof.

Proving vs. Supporting Claims

The first thing to keep in mind is that the phrase "burden of proof" is a bit more extreme than what is often needed in reality. Using that phrase makes it sound like a person has to definitely prove, beyond a doubt, that something is true; that, however, is only rarely the case. A more accurate label would be a "burden of support"—the key is that a person must support what they are saying. This can involve empirical evidence, logical arguments, and even positive proof.

Which of those must be presented will depend very much upon the nature of the claim in question. Some claims are easier and simpler to support than others—but regardless, a claim without any support is not one which merits rational belief. Thus, anyone making a claim that they consider rational and expect others to accept must provide some support.

Support Your Claims!

An even more basic principle to remember here is that some burden of proof always lies with the person who is making a claim, not the person who is hearing the claim and who may not initially believe it. In practice, then, this means that the initial burden of proof lies with those on the side of theism, not with those on the side of atheism. Both the atheist and the theist probably agree on a great many things, but it is the theist who asserts the further belief in the existence of a deity.

This extra claim is what must be supported, and the requirement of rational, logical support for a claim is very important. The methodology of skepticism, critical thinking, and logical arguments is what allows us to separate sense from nonsense; when a person abandons that methodology, they abandon any pretense of trying to make sense or engage in a sensible discussion.

The principle that the claimant has the initial burden of proof is often violated, however, and it isn't unusual to find someone saying, "Well, if you don't believe me, prove me wrong," as if the lack of such proof automatically confers credibility on the original assertion. Yet that simply isn't true—indeed, it's a fallacy commonly known as "Shifting the Burden of Proof." If a person claims something, they are obligated to support it and no one is obligated to prove them wrong.

If a claimant cannot provide that support, then the default position of disbelief is justified. We can see this principle expressed in the United States justice system where accused criminals are innocent until proven guilty (innocence is the default position) and the prosecutor has the burden of proving the criminal claims.

Technically, the defense in a criminal case doesn't have to do anything—and occasionally, when the prosecution does an especially bad job, you will find defense lawyers who rest their case without calling any witnesses because they find it unnecessary. Support for the prosecution claims in such cases is deemed to be so obviously weak that a counter-argument simply isn't important.

Defending Disbelief

In reality, however, that rarely happens. Most of the time, those required to support their claims do offer something—and then what? At that point the burden of proof shifts to the defense. Those who do not accept the support offered must at the very least show just cause for why that support is insufficient to warrant rational belief. This may involve nothing more than poking holes in what has been said (something defense attorneys often do), but it is often wise to construct a sound counter-argument which explains evidence better than the initial claim does (this is where the defense attorney mounts an actual case).

Regardless of exactly how the response is structured, what is important to remember here is that some response is expected. The "burden of proof" is not something static which one party must always carry; rather, it is something that legitimately shifts during the course of a debate as arguments and counter-arguments are made. You are, of course, under no obligation to accept any particular claim as true, but if you insist that a claim isn't reasonable or credible, you should be willing to explain how and why. That insistence is itself a claim which you, at that moment, have a burden to support!