In yesterday's post I wrote of the ways in which burdens of proof can vary and the importance of the burden of production. In summary my point was that not only is the burden of proof on the one asserting a proposition, that party also must meet a minimal burden of production before his assertion should even be considered.
This is similar to Christopher Hitchens' now famous dictum: "That which can be asserted without evidence, can be dismissed without evidence." My point was that Mr. Hitchens was correct but not assertive enough. I think his maxim should be: "That which can be asserted without evidence SHOULD be dismissed without evidence."
I also mentioned that a burden of proof can shift from one party to the other. Depending on the circumstances, there can be evidence that gives rise to a presumption that an asserted fact is true, even if it doesn't prove that fact conclusively. In such cases, the burden of proof will shift to the party asserting that the proposition isn't true.
For instance, if repeated, rigorous scientific studies showed that the adherents of one particular religion had their prayers answered significantly more often than those of other religions (and more often than explicable by random chance--which, of course, is the same thing), then one could argue that a presumption had arisen that this particular religion just might have a line on the truth. Members of that religion could plausibly argue that such evidence gave rise to a presumption that their religion was true. See ftnt.
In such a case, the burden of proof would shift to atheists to prove that this evidence was not evidence of god but of some other explanation. The burden of proof would shift to atheists ONLY with regard to explaining away this evidence, however. It would not shift to atheists with regard to disproving god's existence. For that to happen, there would have to be nearly overwhelming evidence of god's existence.
I've tried to think of an example of the amount of proof for god's existence would be necessary for the burden of proof to shift to atheists on the central question and I haven't been able to do so.
For instance, if that religion's ancient holy writings were completely accurate and had information that humans could not possibly have known at the time and place they were written, there were regular, observable miracles being performed either directly by an apparent god or by members of the favored religion, if god showed himself periodically to fix things and give good advice, etc. Maybe then it could be argued that atheists would have the burden of proof with regard to god's existence.
Even in such extreme circumstances, however, it would still only be the actual evidence that atheists would have to explain because once the evidence is explained we would be back to where we are now: No evidence at all for a very extraordinary proposition.
Why would atheists never have the burden of proof? Because, as I explained before, the god hypothesis is literally impossible to disprove--as is any hypothesis with sentient magical beings at its core.
Yet this shifting of the burden is precisely what theists try to do almost any time one debates them. They do this because, to them, god's existence is "obvious". In their minds "there is no other explanation for the existence of the universe" and "everybody believes" in god. I have refuted these points before: here and here with a further explanation of why the argument from existence fails here.
I mention these points because you should be ready in any debate where the burden of proof is said to be on the atheist to give quick refutations of those two underlying fallacies when they come up because they will if the debate lasts long enough.
Ftnt.: Recently bloggers P.Z. Myers and Jerry Coyne--with Greta Christina on his side--debated whether there was any amount of proof that would convince them to believe in god. I confess, I would be seriously tempted to believe again if there were any remotely plausible evidence such as a series of valid studies showing the efficacy of prayer--even if just for one religion. My re-conversion, however, would be subject to revision or retraction should better atheists than I ever explain away such studies. Interestingly, Myers' take on the issue is similar to mine in this post in that he says the religious haven't even framed a coherent hypothesis to consider. Thus, he sees the notion of god as even further removed from serious consideration than I do.