I’ve finally gotten around to starting Victor Reppert’s book, C.S. Lewis’s Dangerous Idea: In Defense of the Argument from Reason. As Reppert points out in one of his chapters, we can really think of the “argument from reason” as the name for an entire family of theistic arguments. Indeed, Reppert formulates six different versions of the argument from reason which, he says, can be combined to form a cumulative case.

Here are his six arguments.

1. Argument from Intentionality
2. Argument from Truth
3. Argument from Mental Causation
4. Argument from the Psychological Relevance of Logical Laws
5. Argument from the Unity of Consciousness in Rational Inference
6. Argument from the Reliability of Our Rational Faculties

What follows is Reppert’s formulation of each argument.

Argument from Intentionality

1. If naturalism is true, then there is no fact of the  matter as to what someone’s thought or statement is about.
2. But there are facts about what someone’s thought is about. (Implied by existence of rational inference.)
3. Therefore, naturalism is false.

Argument from Truth

1. If naturalism is true, then no states of the person can be either true or false.
2. Some states of the person can be true or false. (Implied by existence of rational inference.)
3. Therefore, naturalism is false.

Argument from Mental Causation

1. If naturalism is true, then no event can cause another event in virtue of its propositional content.
2. But some events do cause other events in virtue of their propositional content. (Implied by existence of rational inference.)
3. Therefore, naturalism is false.

Argument from the Psychological Relevance of Logical Laws

1. If naturalism is true, then logical laws either do not exist or are irrelevant to the formation of beliefs.
2. But logical laws are relevant to the formation of beliefs. (Implied by existence of rational inference.)
3. Therefore, naturalism is false.

Argument from the Unity of Consciousness in Rational Inference

1. If naturalism is true, then there is no single metaphysically unified entity that accepts the premises, perceives the logical connection between them and draws the conclusion.
2. But there is a single metaphysically unified entity that accepts the premises, perceives the logical connection between them and draws the conclusion. (Implied by existence of rational inference.)
3. Therefore, naturalism is false.

Argument from the Reliability of Our Rational Faculties

1. If naturalism is true, then we should expect our faculties not to be reliable indicators of the nonapparent character of the world.
2. But our faculties do reliably reveal the nonapparent character of the world. (Implied by existence of rational inference.)
3. Therefore, naturalism is false.

Since I’ve just started to read his book, I don’t have much to say other than this. Like many people, Reppert suggests that different arguments have more strength when considered as a cumulative case, as opposed to when each argument is considered individually. Also like many people, however, it appears that Reppert doesn’t (1) actually spell out the cumulative case argument or (2) defend the claim that the cumulative case is stronger. In his defense, perhaps he thought it was too obvious. But, if so, I disagree with him. It may be that his six arguments do combine to form a cumulative case, but that needs to be shown and not just asserted. (I trust Victor will correct me if I’m in error and he does do this in his book or elsewhere.)

As I’ve explained before, cumulative case arguments have a very natural and straightforward Bayesian interpretation (scroll down to “Cumulative Case Arguments” in the linked PDF file). The simplest way to explain it is this. Suppose you have two facts, F1 and F2, and you want to show that F1 and F2 combine to form a cumulative case favoring hypothesis H1 over hypothesis H2. How do you do this?

Let’s start with what not to do. You don’t do this by showing:

1. F1 favors H1 over H2.

2. F2 favors H1 over H2.

Instead, you need to show two things:

1. F1 favors H1 over H2.

2. F2 favors (H1 combined with F1) over (H2 combined with F1).

I think it’s an interesting question whether Reppert’s six arguments can be combined to form a cumulative case in this way.

For example, start with intentionality — let’s treat intentionality as F1. (As an aside, I wonder if the fact of intentionality is conditionally independent of the fact of consciousness. In other words, does it even make sense to suppose there is a state of affairs where consciousness exists, but intentionality does not? I don’t know. But let that pass.) Let’s just assume, for the sake of argument, that intentionality favors theism over naturalism. So we have:

1′. F1 favors T over N. [assumption]

Going out of order, let F2 be mental causation. If F1 and F2 are a cumulative case, then we need reason to believe the following is true.

2′. F2 favors (T combined with F1) over (N combined with F1).

The problem is that the truth of 2′ is far from obvious. If we’re including the fact of intentionality in our background knowledge — i.e., if we are already assuming that intentionality exists — then it’s far from obvious that mental causation adds much, if anything, to the case for theism and against naturalism. I’m inclined to believe this: the existence of consciousness is certain on the fact of intentionality. Furthermore, the existence of mental causation is very highly probable, if not certain, on the fact of consciousness. Ultimate metaphysical hypotheses like theism and naturalism don’t seem to change this at all, once we include intentionality (or consciousness) in our background knowledge. Therefore, 2′ is doubtful.

With that said, I must write that I am initially VERY skeptical of the argument from intentionality. I haven’t thought about it deeply, so take my comments with a grain of salt. But because I consider consciousness and intentionality linked, I don’t see how the argument from intentionality adds to the argument from consciousness. In Reppert’s defense, he might say, “It doesn’t. So go deal with the argument from consciousness!” Which would be a fair reply.

Okay, I said these would be some very preliminary thoughts.