I would like some responses to this metamodel for philosophical arguments. This is part of a paper I am writing for the Blackwell Companion to Natural Theology on, you guessed it, the argument from reason.
Before launching into the discussion of the argument from reason, some preamble about what philosophical arguments can be expected to do is in order. To do this we must consider the scope and limits of arguments. What at maximum one can hope for, in presenting an argument, is that the argument will be a decisive argument in favor of one’s conclusion. A decisive argument is an argument so strong that, with respect to all inquirers, the argument is such that they ought to embrace the conclusion. Even when a decisive argument is present, some may remain unpersuaded, but in these are cases of irrationality.
The difficulty here is that by this standard very few philosophical arguments can possibly succeed. This is largely because in assessing the question of, say, whether God exists, numerous considerations are relevant. Since we can concentrate on only one argument at a time, it is easy to get “tunnel vision” and consider only the piece of evidence that is advanced by the argument. But a person weighing the truth of theism must consider the total evidence. So I propose to advance a different concept of what an argument can do. I will assume for the sake of argument, that people will differ as to their initial probabilities concerning the probability that God exists. The question I will then pose is whether the phenomenon picked out by the argument makes theism more likely, or makes atheism more likely. If it makes theism more likely to be true than it would otherwise be before we started thinking about the phenomenon in question, then the argument carries some weight in support of theism. If it makes atheism more likely, then it provides inductive support for atheism.
The model I am proposing is a Bayesian model with a subjectivist theory of prior probabilities. We begin by asking ourselves how likely we thought theism was before we started thinking about the argument in question. We then ask how likely the phenomenon is to exist given the hypothesis of theism. We then ask how likely the phenomenon is to exist whether or not theism is true. If the phenomenon is more likely to exist given theism than it is to exist whether or not theism is true, then the argument carries some inductive weight in favor of theism.
It should be added that one can be an atheist and admit that there are some facts in the world that confirm theism. You can also be a theist and maintain that some atheistic arguments enhance the epistemic status of atheism. Some theists have made just this sort of claim on behalf of the argument from evil. That is, they are prepared to concede that the argument from evil does provide some epistemic support for atheism, but not enough epistemic support to make atheists out of them.