In the interview with Luke Muehlhauser, linked below, we got briefly into the subject of mutual support vis a vis the existence of God and miracles like the resurrection. The question, related to a debate my husband and I have had with Alvin Plantinga, arose (though not in so many words): Isn't the existence of God a premise for belief in any miracle, in which case isn't it circular to say that a miracle increases the probability of the existence of God?
I feel that I wasn't as clear as I could have been in responding to this question, so I'd like to say a little more about the issue of mutual support. Here is the technical paper that addresses this subject, but here I want to try to do this non-technically as much as possible.
Most of us are inclined to reason about things in the following way: Use only propositions for which you have lots and lots of support. Base your predictions on what these propositions really lead you to believe, and go from there.
So, for example, a man whose wife always makes him a chocolate cake for his birthday is most concerned with the question of whether, this year, his wife will continue the tradition. He takes his wife's existence for granted, because it's overwhelmingly supported. He also doesn't worry about whether he's hallucinating all his past apparent memories of chocolate cakes over the years. His only concern is whether she will be too busy this year, and he reasons forward, using what is strongly supported already, to make a prediction about this year's birthday. He may, for example, justifiably conclude that he will get a chocolate cake this year.
In this context, it seems sort of pointless and trivial to argue after the fact that the smell of chocolate cake he notices when he comes home on his birthday is evidence for his wife's existence. No one is worrying about that, and so no one thinks twice about the fact that the evidence runs in that direction.
But if you stop to think about it, it's true: The direct sensory evidence of the baking chocolate cake is evidence that Someone exists who made the cake, especially since he has been gone all day and knows he didn't do it.
Is there any circularity going on here? Not at all. He expected the cake, because he had other evidence from the past both of his wife's existence and of her intention to make the cake. He now has even more evidence of interaction with his wife in the form of the smell of the baking cake. We have, of course, reached a point of diminishing returns here, since the probability for his wife's existence was close to 1 already, even before he noticed that Someone had been in the house baking the cake. To make the epistemic force of the sensory evidence of the cake more vivid, we would perhaps have to imagine the poor fellow as suffering from some sort of mental illness that makes him temporarily doubt that he has a wife, in which case the direct sensory evidence of the chocolate cake would take on a lot more practical importance for him than it has in the ordinary situation!
Here's another example: Suppose that I have a long-distance friend named Bill Smith whom I have never met. Bill and I have talked a number of times on the phone, and the last time we spoke, he asked for my e-mail address and said that he would like to send me a paper. A week or so later, up pops an e-mail in my in-box that purports to be from Bill Smith and that has the paper attached.
Once again, we have evidence here going in two directions but no circularity. The evidence of my previous phone conversations gave me some reason to believe that Bill Smith exists already and also to expect the e-mail. The direct evidence--the "after-the-fact" evidence--that I've actually received an e-mail from someone by that name gives me more reason to believe in the existence of Bill Smith.
And note something that is perhaps clearer in this example than in the cake-baking example: Most of us receive e-mails occasionally from people whom we previously had no contact with. (I certainly have.) Once we have the e-mail, even though we did not have a high previous probability for the existence of the writer or reason to predict the arrival of the e-mail, we have reason to believe in his existence. In other words, the e-mail is evidence for the existence of its author independent of the lowness or highness of our prior probability for the existence of the author.
It's certainly true that my previous good evidence for the existence of Bill Smith and our conversation gave me a higher prior probability for the arrival of an e-mail than I would have had otherwise. But that is not a necessary condition for my believing, on the basis of my sensory evidence and knowledge of e-mail, etc., alone, that I have indeed received an e-mail from a person named Bill Smith (who therefore exists).
In the same way, our prior reasons for believing that God exists (or for doubting God's existence) will be quite relevant to the prior probability of any given miracle. Just as I will not expect to receive an e-mail from a person of whom I have never heard, so I will not expect a miracle from a God for whom I believe there to be little or no positive evidence. On the other hand, if I believe there to be some independent evidence for God's existence, this will raise the prior probability of a miracle. And if I believe there to be enormous evidence--for example, if I believe that the arguments of natural theology render the existence of an omnipotent, omniscient, and omnibenevolent eternal God a certainty--this will "help" the prior probability for a miracle a lot, though it will still not make me positively expect most specific miracles.
But direct evidence for a miracle--for example, testimonial evidence in a context that makes it extremely hard to explain as a lie, etc.--is evidence for the existence of God as well, and it is evidence for the existence of God either way--that is, regardless of whether my prior probability for the existence of God was high or low. Just as the evidence of an e-mail purporting to be from Bill Smith is evidence for the existence of Bill Smith even if my prior probability for both his existence and the coming of the e-mail was low, so it is in the case of miracles and the existence of God.
What this means is that we can have evidential "lines" going in more than one direction without any circularity.
We might be inclined to say that
A: Bill Smith exists
is evidence for
B: I receive an e-mail from Bill Smith
and also that B is evidence for A when the e-mail pops up in my inbox. Drat! A circle! But not really: To say that A is a premise for B is a convenient shorthand, but it comes at the cost of some loss of clarity, and in this type of situation it can be positively misleading. The clearer way to think of it is to think of a set of other evidence, such as
A1: I seem to remember a conversation on the phone with Bill Smith on June 1
A2: I seem to remember a conversation on the phone with Bill Smith on August 25
A3: Bill Smith asked for my e-mail address.
A1-A3 provide one set of evidence that Bill Smith exists and that he intends to send me an e-mail, which raises the the probability of B.
On the other hand,
B1: I have received an e-mail which says that it is from someone named Bill Smith
is evidence for A, independent of whether I have A1-A3. Thus, A1-A3 and B1 are all evidence pertinent to B.
And that's all there is to it.
Look, Ma, no circles!
Now, let's see if I've succeeded in expressing this clearly. What think you, readers?