It is a common contention between theists and atheists as to who has the "burden of proof." Who has this burden - theists? atheists? both? neither? This issue comes up often enough in debates and discussions that it really needs to be addressed here in some detail.
Theism and Evidence That the theists have some burden of proof simply cannot be denied. They are obviously making at least one claim - that at least one god exists. Theists must, then, be prepared to offer justification for their claims - they must face up to their burden of demonstrating that their assertions are reasonable. Of course, it would be unusual for theists to limit themselves to just the claim that a god exists - normally, there will be concurrent and related claims about what this god wants and how we are to live, and those too will require some degree of support from the theist. The more necessary and fundamental they are to the theist's religious belief system, the greater will be the need for support.
How is a theist to support their claims about their god(s)? Well, that will depend entirely upon the nature of the god for which they are making those claims. As with any other claim about any other sort of thing - the nature of the support is dependent upon the nature of the object in question. There isn't any one set of catch-all "proofs" which will suffice for every possible god. Clearly, then, one of the first steps any theist will have to take is to explain the nature of this god they are claiming. What is it, exactly?
Unless we have a good idea of what we are looking for, we'll never know if we've found it or not! Unless a person's theism is literally nonsense, they must be capable explaining the content of their belief.
Parallel to this, and well recognized by many theists, is the fact that the more ambiguous and vague the deion of their god is, the easier it will be to find "evidence" for it. If they don't start out clear about what they are describing, then they can later add on anything they wish as "support." No critically and logically thinking atheist should accept this. If we do not have a clear idea of what "god" mean, then the statement "god exists" is literally communicating nothing to us - and there will be no reason to accept it as rational, much less true.
Atheism and Evidence But what about atheists? What burden of proof do they hold? Well, we should first notice that atheists aren't necessarily making any particular claims about the world. When a person says to you "I am an atheist," all you can really assume is that they are saying "I do not believe in any gods." That isn't much, and unless someone wishes to argue that the atheist is mistaken or lying about their beliefs, then they should be taken at their word. They do not believe, and that's pretty much that.
Of course, that isn't going to be the limit of the average atheist's beliefs and nonbeliefs. For example, an atheist might deny the existence of some gods - and that denial is itself a claim which warrants justification and support, if it is questioned. Other atheists might not deny outright a particular god, but will deny certain associated claims made in relation to belief in that god. For example, an atheist might assert that an alleged god might exist - but that if it does, it cannot be omnibenevolent. Or an atheist might accept that, if a god exists, it certainly didn't create the world in seven days.
Again, such assertions should be supported if questioned. Common to all is a fundamental connection to atheism. They aren't necessary to atheism - no one need deny any particular theistic beliefs in order to be an atheist, they only need to not believe in any gods, whatever their reasons or attitudes. But the aforementioned claims are clearly related to atheism, since they involve gods.
Quote of the week:
My presumption of atheism is closely analogous to the presumption of innocence in the English law;
[...]
...the onus of proof...is up to the theist: first, to introduce and defend his proposed concept of God; and, second, to provide sufficient reason for believing that this concept of his does in fact have an application.
Antony Flew, God, Freedom, and Immortality. They are also probably important to the atheism of the individual atheist in question, and that makes the ideas worth supporting if someone is attempting to justify their atheism. Other beliefs which an atheist might hold and which are not necessary for their atheism, like a belief that the world is round, hardly need any sort of support in the context of debating atheism and theism. Just because an atheist believes something doesn't mean that that belief is so connected to atheism that, in order to justify atheism, the belief in question needs to be justified. The same goes with theism.
Sadly, not all theists seem to recognize this. Some take an attitude that anything an atheist believes and which they do not believe must be fundamental to atheism, and, hence, must be justified by the atheist. If the atheist cannot or chooses not to, then the theist will quickly claim an unwarranted "victory" since "atheism" has not been "justified." But once we realize how and why the belief in question is not necessary to atheism, then we must also realize that no matter what happens to this belief, atheism itself need not be affected.
Atheism and Evolution One common example of this is the issue of evolution. Religious fundamentalists who hope to question whether or not atheism is reasonable will often and quickly reach to one of their favorite debating points: evolution. Evolution is probably the central defining idea at the heart of the modern biological sciences. As such, it is probably the central target of the fundamentalist reaction to modernity and science. For some reason, they cannot seem to get away from evolution - one could say that many are almost preoccupied by it - and so atheists must be prepared to deal with the common fundamentalist question: "So, where did life come from?"
The first reaction, of course, is that this question has nothing at all to do with evolution. The theory of evolution describes how life has developed over time, not how it originated. Few fundamentalists understand this - probably because they have spent too much time reading creationist propaganda and zero time reading any actual science texts. If debate gets beyond this issue and ever addresses what evolution really is, the next reaction is: what's the point? Unless an atheist does not believe in any gods because of evolution, the an atheist has absolutely no need to defend evolution in order to defend atheism.
Although I personally feel that evolution is fatal to most traditional Western theistic ideas, it is clear that evolution and belief in god(s) are separate issues for most people. There are theists who accept the explanation of evolution, and theists who do not. There are atheists who accept the explanation of evolution, and atheists who do not. If evolution is incompatible with belief in any gods, it is only going to be incompatible with very particular conceptions of god(s) - but atheism involves not believing in any gods, not just a few.
So atheism is not inherently dependent upon evolution and, hence, no atheist is automatically required to defend it. Evolution could be entirely wrong without atheism being touched. Fundamentalists, however, are very theistically prejudiced - they only ever think about their own god and totally disregard every other. They cannot think outside of their own narrow theological box - and so if some idea is incompatible with their belief in their god, then it must be incompatible with theism in general. Ridiculous, but that's the way it is.
Atheism and the Universe Discussion might proceed to origins in general - namely, the origin of the entire universe. Theists will commonly ask "Well, where did the universe come from?" Like the atheist reaction to evolution, we can approach this question with: "I don't know. So what?" Unless a person's atheism is dependent upon a particular deion of the origin of the universe, they neither need to know the answer to the theist's question nor do they need to support any particular answer. Like evolution, the question of the origin of the universe might have a relation to a person's atheism, but need not support it. Both are further beliefs which go beyond simple nonbelief in gods - but so is the belief that the Earth is round. An atheist need not support and justify any of them in an effort to support or justify their atheism.
Just because a theist's beliefs regarding those issues are central to their theism doesn't mean that the atheist must oblige them and make those issues central to her atheism. If a theist wishes to make the question of the origin of the universe fundamental to their belief in a god - fine. That will then be one more thing which they will have to explain in their effort to support belief in their god. But that has no impact whatsoever in any effort I might make to support my not believing in their god. The only possible origin for the universe which is incompatible with my atheism is that of a creation by a god. This, of course, would be for the theist to demonstrate - and if they cannot, my atheism remains, whatever the real origin is. I do not need to account for this "real origin" in order to account for my atheism.
Theists need to explain and account for their god, because that's what theism is: belief in a god. Their claims of theism are about their god, so that is what they have to talk about. I do not, as an atheist, automatically need to explain or account for anything in particular in the universe because my atheism is not about the universe. I hold a wide variety of beliefs - and my atheism is dependent upon just a few of them. The rest could be absolutely wrong without automatically changing the fact that I do not believe in any gods. So, theistic demands that I do justify them are merely efforts to distract from the issue: whether or not their god exists and whether or not they can support their claims.
Atheism and Disproof Now, are atheists required to disprove theistic claims? In general, no. If an atheist feels that she can disprove some particular theistic claim, then she should feel free to move forward and do so - but she should not feel that she is laboring under any obligation. At the minimum, the atheist needs to examine whatever support is offered for theistic claims and show how and why they might be inadequate for the job they are assigned. If the theist, however, does not offer any support - there is little more that the atheist can do but ignore them. If they do not wish to offer rational support for their claims, then they have no interest in engaging in rational discussion on the issues - and what else is left?
So the atheist committed to rational beliefs and rational discussion has a relatively clear course before them. First, the burden of proof is on whoever is making the claims. This is always at least the theist, since they are by definition making at least one claim about at least one god. This is sometimes on the atheist, if they choose to deny something specific.
Second, the justification of atheism, if required, can only proceed from whatever is fundamental and necessary to atheism. As long as atheism is simply nonbelief, that means that little or no justification is required, at least early on. Only after the theist has presented coherent and rational arguments might the atheist need to explain why she does not accept them. At that point, justification of atheism is based upon inadequate justification for theism.
Moreover, the atheist should not be maneuvered into justifying other beliefs which themselves are not necessary to their atheism. Just because the atheist happens to disagree with the theist on other issues does not mean that the atheist needs to justify these other beliefs in order to justify atheism. The atheist does not have any more automatic need to justify acceptance of evolution than she does of justifying acceptance that the Earth is round or that the death penalty is just.
Instead of allowing the discussion to go off on irrelevant tangents amenable to the theists personal obsessions, the atheist must strive to keep the discussion focused. If the discussion is about the existence of gods, then that is where it must be kept. Since the burden of proof is primarily upon theists, diversions will primarily benefit them - which may in fact be why so many theists go off on tangents, but I cannot say for sure.