Background: In the context of a review of Dan Barker’s book, Godless, Randal Rauser had a very brief, even cryptic, exchange in the combox for his about God-of-the-Gaps (GOTG) arguments. (See here and here.) That exchange led to his latest post, which you can read for yourself here. I’ve decided to post my response on my own blog here, with some edits for further clarification.

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I haven’t read Barker’s book, so I can only comment on what you have quoted:

“Many of these [theistic] arguments are reduced to a ‘god of the gaps’ strategy. At most, the theists might prove the existence of a current gap in human knowledge, but this does not justify filling the gap with their god. After all, what happens when the gap closes someday? The gaps are actually what drive science–if we had all the answers there would be no more science.” (Godless, 104-5)

Let’s start with the ‘god of the gaps’ (GOTG) strategy. What is a GOTG argument and why are such arguments so bad?

Theistic Argument Schema #1 (Focus on Gap in Scientific Knowledge)

GOTG arguments go like this.

(1) There is some fact, F, which science cannot explain today (in terms of naturalistic, mechanistic, unguided) causes

(2) [probable] Science will never explain be able to explain F. [inductive inference from 1]

(3) The existence of God does explain F.

(4) Therefore, the existence of God is the best explanation for F. [from 2 and 3]

(5) [probable] God exists. [inductive inference from (4)]

The key feature of schema #1 (and other schemas like it) is that “science cannot explain F today” plays a major role in the argument.

The move from (1) and (2) is weak. Science has been extremely successful in explaining a wide variety of phenomena in terms of naturalistic, mechanistic causes. Before we even get into the specifics of F, it’s already extremely likely that F has a naturalistic, scientific explanation. In Bayesian terms, “F has a naturalistic explanation” has a high prior probability. This is why I agree with the Barker quotation.

Theistic Argument Schema #2 (Focus on Content of Propositions)

(1) There is some fact F, we know to be true.

(2) The content of the proposition, “The mental exists and, if anything physical exists, explains why anything physical exists” (hereafter, “source idealism”), provides us with reason to expect F or, if it doesn’t provide a reason to expect F, makes F less surprising than it would be on “source physicalism.”

(3) The content of the proposition, “The physical exists and, if anything mental exists, explains why anything mental exists,” (hereafter, “source physicalism”), provides no reason to expect F (or it provides some reason, but less of a reason than what “source idealism” provides).

(4) Therefore, we’d expect F more on the assumption that source idealism is true than on the assumption source physicalism is true.

(5) [probable] Source physicalism is false.

The key feature of schema #2 (and other schemas like it) is that “science cannot explain F today” plays no role whatsoever in the argument. Although F might, indeed, be a fact that science has no explanation for, the lack of scientific explanation for F does zero work in the argument. (In fact, the lack of a scientific explanation for F isn’t even a premise in the argument!) What does do the work in the argument? The content of the propositions represented by the labels “source idealism” and “source physicalism.”

This is a major advantage of schema #2 over schema #1: because “science cannot explain F today” plays no role whatsoever in the argument, schema #2 makes an objection based on the history of science irrelevant. If I were a theist trying to make an argument for God’s existence based one some fact F–a fact which in principle could have a scientific explanation but currently does not (such as fine-tuning, origin of life, consciousness, free will, etc.)– I would use schema #2, not schema #1.

Example: An Argument from Consciousness Using Schema #2

For example, suppose we decide to adopt schema #2 as a “template” for theistic arguments (specifically, natural theology) and we want to try it out with consciousness. This would yield something like the following.

(1) Consciousness exists.

(2) The content of the proposition, “The mental exists and, if anything physical exists, explains why anything physical exists” (hereafter, “source idealism”), provides us with reason to expect consciousness or, if it doesn’t provide a reason to expect consciousness , makes consciousness less surprising than it would be on “source physicalism.”

(3) The content of the proposition, “The physical exists and, if anything mental exists, explains why anything mental exists” (hereafter, “source physicalism”), provides no reason to expect consciousness.

(4) Therefore, we’d expect consciousness much more on the assumption that source idealism is true than on the assumption source physicalism is true.

(5) [probable] Source physicalism is false.

In this case, source idealism not only ‘predicts’ that something mental exists, but it says that something mental explains the existence of everything physical. In other words, something irreducibly mental plays a ‘deep’ role in a theistic worldview. In contrast, source physicalism is logically compatible with the nonexistence of anything mental. If source physicalism is true, the only want to ‘get’ something mental is to have living organisms with bits of matter arranged in very specific and complex ways (e.g., organisms with brains or something very much like a brain). But source physicalism is logically compatible with all sorts of scenarios where such bits of matter never get into that kind of arrangement. For example, source physicalism is logically compatible with a possible world in which only one universe exists, the universe allows carbon-based life, carbon-based life arises through naturalistic abiogenesis mechanism, and then evolution never progresses past single-celled life. Source physicalism is also logically compatible with a similar possible world, but with no life whatsoever. And so on.

In a source physicalism world, mental phenomena like consciousness do not play the kind of ‘deep’ role that they play in a source idealism world. (That’s the whole point of source physicalism.) And so the existence of mental phenomena like consciousness–even if consciousness turns out to have a naturalistic, scientific explanation–is very surprising on source physicalism but expected on source idealism.

The ‘Catch’

If a theist decides to use schema #2, however, there is a catch: in order to maintain logical consistency, the theist is required to admit that there are good, parallel arguments against source idealism and for source physicalism.

For example, notice the symmetry in the definitions of source idealism and source physicalism: one is based upon the mental and the other based upon the physical. You might say that the argument from consciousness described above is a version of an argument family we can call ‘arguments from mentality.’ Source physicalists have a corresponding argument family of their own, what we can call ‘arguments from physicality.’ Similar to the argument I defend in this post, source physicalists can argue as follows.

(1) Matter exists.

(2) The content of the proposition, “The physical exists and, if anything mental exists, explains why anything mental exists” (hereafter, “source physicalism”), provides us with reason to expect matter or, if it doesn’t provide a reason to expect matter, it makes the existence of matter less surprising than it would be on “source idealism.”

(3) The content of the proposition, “The mental exists and, if anything physical exists, explains why anything physical exists” (hereafter, “source idealism”), provides us with no reason to expect expect matter.

(4) Therefore, we’d expect matter much more on the assumption that source physicalism is true than on the assumption source idealism is true.

(5) [probable] Source idealism is false.