The Argument from Reason as a Theistic Argument

III. The Argument from Reason and Natural Theology
We might ask the following question: In what sense is the argument from reason a piece of natural theology. The job of natural theology is supposed to be to provide epistemic support for theism. However, the argument from reason, at best, argues that the ultimate causes of the universe are mental and not physical. This is, of course, consistent with various world-views that other than traditional theism, such as pantheism or idealism.
It's a good idea to look at what happened in the case of the argument from reasons’s best-known defender, C. S. Lewis, to see how the argument contributed to his coming to belief in God. Lewis had been what was then called a "realist", accepting the world of sense experiece and science as rock-bottom reality. Largely through conversations with Owen Barfield, he became convinced that this world-view was inconsistent with the claims we make on behalf of our own reasoning processes. In response to this, however, Lewis became not a theist but an absolute idealist. It was only later that Lewis rejected absolute idealism in favor of theism, and only after that that he became a Christian. He describes his discussions with Barfield as follows:
(He) convinced me that the positions we had hitherto held left no room for any satisfactory theory of knowledge. We had been, in the technical sense of the term, “realists”; that is, we accepted as rock-bottom reality the universe revealed to the senses. But at the same time, we continued to make for certain phenomena claims that went with a theistic or idealistic view. We maintained that abstract thought (if obedient to logical rules) gave indisputable truth, that our moral judgment was “valid” and our aesthetic experience was not just pleasing but “valuable.” The view was, I think, common at the time; it runs though Bridges’ Testament of Beauty and Lord Russell’s “Worship of a Free Man.” Barfield convinced me that it was inconsistent. If thought were merely a subjective event, these claims for it would have to be abandoned. If we kept (as rock-bottom reality) the universe of the sense, aided by instruments co-ordinated to form “science” then one would have to go further and accept a Behaviorist view of logic, ethics and aesthetics. But such a view was, and is, unbelievable to me. C. S. Lewis, Surprised by Joy (San Diego, Harcourt Brace, 1955), 208.

Lewis did not, however, embrace theism at this point. Instead, he opted for Absolute Idealism, a philosophy prevalent in Oxford in the 1920s, although it is not widely held today. He wrote of this again in Surprised by Joy:

It is astonishing (at this time of day) that I could regard this position as something quite distinct from Theism. I suspect there was some willful blindness. But there were in those days all sorts of blankets, insulators, and insurances which enabled one to get all the conveniences of Theism, without believing in God. The English Hegelians, writers like T. H. Green, Bradley, and Bosanquet (then mighty names), dealt in precisely such wares. The Absolute Mind—better still, the Absolute—was impersonal, or it knew itself (but not us?) and it was so absolute that it wasn’t really much more like a mind than anyone else….We could talk religiously about the Absolute; but there was no danger of Its doing anything about us…There was nothing to fear, better still, nothing to obey.

Nevertheless, further considerations drove Lewis out of idealism into theism. He wrote:

A tutor must make things clear. Now the Absolute cannot be made clear. Do you mean Nobody-knows-what, or do you mean a superhuman mind and therefore (we may as well admit) a Person? After all, did Hegel and Bradley and all the rest of them ever do more than add mystifications to the simple, workable, theistic idealism of Berkeley? I thought not. And didn't Berkeley's "God" do all the same work as the Absolute, with the added advantage that we had at least some notion of what we meant by Him? I thought He did. So I was driven back into something like Berkeleyanism; but Berkeleyanism with a few top dressings of my own. I distinguished this philosophical "God" very sharply (or so I said) from "the God of popular religion." There was, I explained, no possibility of being in a personal relation with Him. For I thought He projected us as a dramatist projects his characters, and I could no more "meet" Him, than Hamlet could meet Shakespeare. I didn't call Him "God" either; I called Him "Spirit." One fights for one's remaining comforts.
So did the argument from reason that Lewis accepted make theism more likely in his mind? It certainly did. In his mind it gave him a reason to reject his previously-held naturalism. Now you might think of Absolute Idealism an atheistic world-view, but is does deny the existence of the theistic God as traditionally understood. However the playing field was now considerably narrowed.

Consider the following argument:
1. Either the fundamental causes of the universes are more like a mind than anything else, or they are not.
2. If they are not, then we cannot make sense of the existence of reason.
3. All things being equal, world-views that cannot make sense of the existence of reason are to be rejected in favor of world-views that can make sense of the existence of reason.
4. Therefore, we have a good reason to reject all worldviews reject the claim that the fundamental causes of the universe are more like a mind than anything else. 

Now if you want to hold out the idea that a idealist world-view is nevertheless atheistic, then my argument merely serves to eliminate one of the atheistic options. But suppose someone originally thinks that the likelihoods are as follows:
Naturalism 50% likely to be true.
Idealism 25% likely to be true.
Theism 25% likely to be true.

And suppose that someone accepts a version of the argument from reason, and as a result naturalism drops 30 percentage points. Then those points have to be divided amongst theism and idealism. Therefore the epistemic status of theism is enhanced by the argument from reason, if the argument is successful in defeating naturalism.