This interview between Esteemed Husband and Dale Tuggy on the Trinities podcast contains a lot of good stuff. (Yes, I know that Tuggy is not a trinitarian. Yes, I acknowledge the irony and weirdness of having a podcast named "Trinities" hosted by a non-trinitarian. No, that isn't relevant to this post nor to the content of this particular podcast.)
I have to admit up front that I have not yet listened to the entire podcast myself but only to the first twenty minutes of it. It takes me a while to listen all the way through podcasts, for some reason.
But a really interesting issue comes up in those first twenty minutes (indeed, in the first seven minutes) that is worth highlighting in itself.
Naturalists will often try to do a kind of crude "subtraction" simplicity argument against the existence of God and for naturalism, and it just stinks as an argument. To be clear, I am saying nothing against simplicity as a criterion of theory choice. It is ubiquitous and important. It is what shows us that conspiracy theories are ridiculous. We tacitly use simplicity considerations constantly.
What Esteemed Husband is speaking out against here is a double abuse of simplicity.
1) Reduce simplicity considerations in theory choice to a crude "subtraction of entities" notion, where "entities" does not include things like thoughts, intentions, actions, and coincidences.
2) Place an irrational amount and type of weight on those crudely defined "simplicity" considerations, so that one treats it as argumentatively legitimate to dismiss or explain away positive evidence for the existence of an entity on the grounds that one's resulting "ontology" is "simpler."
Tim's Abraham Lincoln example here (in the early minutes of the interview) is particularly instructive. Suppose that I take my "ontology of entities" and simply subtract Abraham Lincoln from those entities that I believe exist. But I still have the same evidence for the existence of Abraham Lincoln. Is this reasonable to do? Can I justify the resulting bizarre lengths to which I will have to go to explain away the evidence for the existence of Abraham Lincoln on the grounds that there is one entity the fewer in the world described by these conspiracy theories and hence that they are more probable than the theory that Lincoln existed? Obviously not. "One entity the fewer" doesn't translate into "simpler overall" and certainly doesn't translate into "more probable on the evidence that I have."
In fact, any theory that tries to explain the evidence I have for the existence of Lincoln while denying his existence will be far more complex than the theory that Lincoln existed. In the place of the one man, Lincoln, I will have to hypothesize a large number of intentions to deceive, coincidental confusions and mistakes, etc., on the part of innumerable other people, all converging to appear erroneously to support the existence of Lincoln. A-Lincolnism is by no means a simpler theory, and it certainly is not more probable than Lincolnism! But, if I attribute these deceptions and errors all to persons who really existed (other than Lincoln), and if I don't consider thoughts and intentions to be separate entities in an ontological list, then the "list of ontological entities" is shorter by the absence of one entity--Lincoln. Whoop-de-do.
This is very much what the naturalist is doing. The naturalist/atheist makes a great song and dance about the "greater simplicity" of his atheism, but that is really quite meaningless and pointless in the presence of evidence for the existence of God.
Simplicity considerations are always relevant to our actual knowledge of what exists only by being comparative considerations applied to candidate explanations of existing evidence. None of us sits around in a literal evidential vacuum counting up entities.
The question of whether God exists arises because there is some putative evidence for the existence of God, just as the question of whether Lincoln exists would never have arisen in the first place if there weren't (taken to be) some reason to think that Lincoln exists. (The same argument can be applied to the existence of your mother or your best friend.)
Hence, simplicity should be thought of in actual practice as a three-place (at least) relation: Theory A is simpler than Theory B (or theories B-Z) as an explanation of evidence E, which I possess.
As Tim points out (a very important point), there is no general rule in probability theory that, relative to some body of evidence, (A & B) has lower probability than (A & ~B). People are sometimes confused about this from the fact that (A) alone is strictly more probable than (A & B). But it simply does not follow that (A & ~B) is more probable than (A & B). Asserting the negation of B is itself a theoretical commitment that takes on its own theoretical risk and may be improbable relative to the evidence at hand. (And to cover agnosticism, asserting the metalevel claim "The probability of B is no greater than some ceiling n" is also a commitment that may be wrong, given the actual evidence. It is not the same thing as merely asserting A.)
Agnostics and atheists harp on a faux simplicity argument against the existence of God, and sometimes Christians are thrown by it. I think that they may be thrown because they think that the correct way to proceed is to go back to some hypothetical pre-evidential state (that no real person is ever in), to "award points" to an hypothesis that has fewer entities than another hypothesis, then to move forward to take the evidence on board while always continuing to award these extra points (and they are usually supposed to be large amounts of points) to the hypothesis with the fewer "entities." (Again, with a very restrictive notion of "entities.")
But we would never do this in the case of the existence of Lincoln, your mother, zebras, or any other actual entity. We would never sit around in angst over how many points we should award to the hypothesis that there are no zebras, which would in turn allow us to explain away large swathes of positive evidence for zebras. We would never do that for the existence of Lincoln, either. Any consideration of additional ontological "weight" that we take on with the existence of Lincoln or zebras is obviously wholly swamped by the consideration that we would have to compass land and sea (hence radically violating simplicity norms) in order to do without Lincoln, zebras, or Mom in our ontology while still explaining the evidence.
Whether or not the positive evidence for the existence of God actually works to support God's existence is, of course, a separate matter that has to be examined by actually looking at the evidence. (A point Tim emphasizes repeatedly in the interview.) In the course of that evaluation, naturalists should not be given a free pass to keep running back to some unreal, pre-evidential situation and awarding their atheism special points for not having God in its ontology. Evidence evaluation just doesn't work that way.