I have added new material to this post.
In this exchange, David Marshall asks for a debate concerning the Outsider Test for Faith, and Peter Boghossian refuses to begin such dialogue in because Marshall does not give an adequate answer to the question "What would it take for you to lose your faith?"
What Boghossian is applying was defined by Matt McCormick as The Defeasibility Test. The claim here is that unless the believer is willing to indicate what kinds of considerations would cause him to lose his faith, discussion is useless and the believer should be regarded as terminally irrational. This has also been endorsed by Loftus.
Now this, to some, has a reasonable ring to it, harking back to Flew's Falsification Challenge. But I am going to argue that the way it is being employed by McCormick and Boghossian is misguided.
First, I am convinced that there are three factors involved when people make religious decisions. The first is their evaluation of publicly available evidence, evidence that we can all examine. This would be the usual set of reasons we all talk about in the philosophy of religion: the theistic arguments, the problem of evil, the problem of hiddenness, etc. The second factor is one's personal experience. This will differ from person to person and is not available for public inspection. The third is the pragmatics of belief. Some people might be very adversely affected by becoming an atheist, or a theist, and those factors are also relevant for people to consider when they are trying to decide what to believe.
First of all, suppose someone has indicated that they think that there are good theistic arguments, but they also think they would remain a fideistic believer if those arguments were shown to be faulty. If they are in fact bad arguments, wouldn't showing that this is so be worthwhile? Is the only goal of dialogue conversion to atheism?
Second, if I am right, not all of the considerations that go into a reasonable person's choice as to whether or not to be a believer are open for public debate. If I, for example, had a direct experience of God, I can't cause you to have one, too. All I can do is testify to my own experience, and you may or may not believe me.
Plantinga's Purloined Letter example is relevant here. All the public evidence may support the claim that I stole a damaging letter, but I may nevertheless know perfectly well that I didn't steal it.
Third, not all considerations with respect to one's own beliefs with respect to God are even introspectively obvious. If we had asked a subsequent de-convert what it would take for them to give up their faith, I am not sure they could have predicted the scenario that led them to change their mind.
Fourth, some atheists are committed to indefeasibility. Are they terminally irrational?
It is an important conviction of mine that discussion and defense of what I believe is not primarily aimed at the conversion of my discussion partner. I know perfectly well that reasons on both sides of the issue of something like God are far more complex that what we can encompass in a single discussion. Many of my most gratifying discussions with people with opposing views have largely been taken up with the descriptive task, that is, getting and giving a clearer understanding of the relevant issues and our respective positions than we had going in.
I remember once giving my first philosophy paper at the Pacific division of the APA. When I returned to the Pacific APA an undergraduate student came up to me and gave me a paper he had written for an undergraduate philosophy journal, which had what I would now recognize as an "internet infidel" flavor to it. I sent him several paragraphs critiquing what I took to be the naive philosophy of science that his paper embodied. I heard nothing from him for few years, and then received an e-mail indicating that he had become a Christian, and thanking me for my courteous response. Humbling, surely, but I had no idea that this would happen, nor was I especially concerned about trying to convert him.
I am inclined to agree with Lewis that apologetic discussion is about following the argument where it leads. It is not about judging our opponents, or persuading them. Those sorts of transformations involve far more than intellectual assent, though assent is involved and reasons are relevant. But I am not going to be trapped in a version of "What arguments do I have to win with you to make you agree with me." Since it hasn't happened, I don't know what I would do if I saw the evidence differently. And neither do you.