The Evidential Argument from Mind-Brain Dependence: A Reply to Bilbo

In my response to Victor Reppert’s anti-naturalistic argument from pain, I stated that a more specific fact (consciousness is dependent upon the physical brain) about consciousness is antecedently more likely on naturalism than on theism. Bilbo provided several feedback messages in response to this claim. As I read him, he thinks the argument is no threat to Christian theists. It is apparently his view that mind-brain dependence is at least as probable on Christian theism as on naturalism, since Christian theists believe in specific, sectarian doctrines that raise the probability of mind-brain dependence. Since I think Bilbo is wrong about this, I’ll explain why.

First, here is the logical form of my evidential argument.

Let D = the mind is dependent upon the physical brain.
Let T = classical theism
Let N = metaphysical naturalism

(1) D is known to be true.
(2) T is not much more probable intrinsically than N.
(3) Pr(D/N) > Pr(D/T).
(4) Other evidence held equal, T is probably false.

Note the modest nature of this argument. It doesn’t claim that theism is probably false; it merely claims that evidence (D), by itself, favors N over T. This distinction is crucial since it allows for the possibility that there could be other evidence that both favors T and outweighs the evidence for N provided by D. Thus, D should be understood as a prime facie reason for rejecting T.

Let me now turn to some of Bilbo’s specific comments.

Bilbo: “My first problem is that it seems more accurate to say that minds *are* the physical brain on Naturalism (not merely dependent on them). And since this has not been demonstrated, it would beg the question to assert that the postulate that “minds are the physical brain” provides evidence for Naturalism.”

That would indeed beg the question, which is why I never said that! My argument is an inductive argument. It would be illegitimate to state the evidence in such a way that entails the hypothesis to be proved. Therefore, I’ll stick with my original formulation of the premise (“the mind is dependent upon the physical brain”).

Bilbo: “But the Christian views I mentioned also make it antecedently probable that minds will be dependent on the brain. Let’s use the structure of your argument with Christian non-reductive physicalism (hereafter NRP)…”

If theism is improbable given D, then so is Christian theism (or any other more specific belief system that entails theism). Christian theism entails theism; therefore it cannot be more probable than theism. Premise (4) entails that, other evidence held equal, Christian theism is probably false.

I don’t deny the potential relevance of sectarian doctrines to the issue of whether my argument is sound. They could raise Pr(D/T) or lower Pr(D/N). In order to assess the evidential significance of such doctrines, we would need to apply a principle that Draper calls the “weighted average principle” (WAP). Let H represent some Christian doctrine. Then WAP can be represented as follows.

Pr(D/T) = Pr(H/T) x Pr(D/T&H;) + Pr(~H/T) x Pr(D/T&~H)

This formula is an average because Pr(H/T) + Pr(~H/T) = 1. It is not a simple straight average, however, since those two values may not equal 1/2.

Let us consider, then, Christian non-reductive physicalism (CNRP), which Bilbo proposes as a specific doctrine that he believes raises the probability of D given (Christian) theism. Bilbo defines CNRP as “the non-existence of all supernatural beings, except for those that are or were at some point divine (angels, demons, God, etc.).” Bilbo claims that CNRP “entails the nonexistence of disembodied human minds.” Using WAP, we obtain the following.

Pr(D/T) = Pr(CNRP/T) x Pr(D/T&CNRP;) + Pr(~CNRP/T) x Pr(D/T&~CNRP)

In order to reject my evidential argument, therefore, Bilbo would need to show that CNRP raises Pr(D/T) so that it is greater than or equal to Pr(D/N) by using the above formula. Does CNRP do that? I shall leave the question as an exercise for the reader.

(Note: I have borrowed heavily from ideas in another paper by Paul Draper, “More Pain and Pleasure: A Reply to Otte.” I am, of course, responsible for any errors in my post.)