The Evidential Argument from Physical Minds (APM)

Jeffery Jay Lowder | June 13, 2012

Last edited: 13-Jun-12 8:20PM PDT

Informal Statement of the Argument

Scientific evidence shows that human consciousness and personality are highly dependent upon the brain. In this context, nothing mental happens without something physical happening. That strongly implies that the mind cannot exist independently of physical arrangements of matter. In other words, we do not have a soul. And this is exactly what we would expect if naturalism is true. But if theism is true, then souls or, more generally, minds that do not depend on physical brains, are a real possibility. Also, if theism is true, then God is a disembodied mind; God’s mind is not in any sense dependent on physical arrangements of matter. But if nothing mental happens without something physical happening, that is evidence against both the existence of souls and the existence of any being who is supposed to have a disembodied mind, including God. Therefore, the physical nature of minds is unlikely if theism is true, but what we would expect if naturalism is true.

Formal Statement of the Argument

B: The Relevant Background Information
1. Human beings exist. All healthy human beings have minds, including rich conscious experiences and personalities.

E: The Evidence to be Explained
1. Human minds are dependent upon the physical brain.

Rival Explanatory Hypotheses
T: classical theism
N: metaphysical naturalism

The Argument Formulated

(1) Pr(E | B & N) > Pr(E | B & T).
(2) E is known to be true.
(3) T is not much more probable intrinsically than N.
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(4) Therefore, other evidence held equal, T is probably false.

Defense of (1)

The neural correlates of consciousness are logically compatible with T; in other words, it is logically possible that both God exists and human minds are dependent upon physical brains. That is why the argument from physical minds is an evidential argument, not a logical one.

There are are two reasons why mind-brain dependence is antecedently more probable on the assumption that N is true than on the assumption that T is true. First, N entails that, if minds exist, they will be embodied, i.e., Pr(E | B & N) = 1. Naturalism is logically incompatible with disembodied minds, e.g., souls, ghosts, spirits, demons, angels, gods, God, etc. In contrast, T is logically compatible with the existence of both embodied and disembodied minds. Thus, Pr(E | B & T) < 1. Second, disembodied minds are not just a ‘theoretical’ possibility on T; T entails the existence of at least one disembodied mind, namely, God’s mind. Thus, T provides at least some antecedent reason to expect human minds to be disembodied.

Defense of (2)

Following Michael Tooley, I will summarize the evidence for E as follows.

E1.1. When an individuals brain is directly stimulated and put into a certain physical state, this causes the person to have a corresponding experience.
E1.2. Certain injuries to the brain make it impossible for a person to have any mental states at all.
E1.3. Other injuries to the brain destroy various mental capacities. Which capacity is destroyed is tied directly to the particular region of the brain that was damaged.
E1.4. When we examine the mental capacities of animals, they become more complex as their brains become more complex.
E1.5. Within any given species, the development of mental capacities is correlated with the development of neurons in the brain.

Comments on (4)

Note that while the above argument implies that we have a good prima facie reason to believe that T is probably false (since T and N are incompatible), it does not imply that we have a good prima facie reason to believe that N is true (since T and N are not jointly exhaustive and so could both be improbable). So the argument from physical minds could be more accurately described as an argument against T than as an argument for N, though of course in some sense it is both.

Objections to APM

Objections to (1)

Objection: Even if there are no souls, it’s absurd to say that God can’t exist with a disembodied mind merely because humans don’t exist without a disembodied mind.

Reply: Yes, that would be absurd, which is why I don’t argue that God can’t exist because of human mind-brain dependence. In the jargon of philosophy of religion, my argument is an evidential argument from physical minds, not a logical argument from physical minds.

Objection: But consciousness is evidence for theism, so APM must be wrong.

Reply: Consciousness may well be evidence for T, but this is not of obvious relevance to APM, which states that, given human consciousness exists, the fact that is dependent upon the physical brain is evidence favoring N over T. In fact, to deny that mind-brain dependence can be evidence for N because consciousness is evidence for T is to come dangerously close to the fallacy of understated evidence.

Objection: According to eliminative materialism, what we’d expect if naturalism is true is that we don’t even have mental states.

Reply: Like the previous objection, this objection is not of obvious relevance to APM and for the same reason. The existence of mental states is included within the background information for APM. At best, this objection, like this previous objection, provides support for an independent (though related) argument from consciousness for T.

Objection: But most Christians are non-reductive physicalists. Thus, on T we do have antecedent reason to expect mind-brain dependence and so (1) is false.

Reply: If T is improbable given E, then so is Christian theism (or any other more specific belief system that entails T). Christian theism entails T; therefore it cannot be more probable than T. Premise (4) entails that, other evidence held equal, Christian theism, along with every other version of T, 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(E | T) or lower Pr(E | 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.

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 the objector proposes as a specific doctrine that he believes raises the probability of E given (Christian) theism. Using WAP, we obtain the following.

In order for

CNRP to provide a defeater for (1), therefore, the objector would need to show that CNRP raises Pr(E | T) so that it is greater than or equal to Pr(E | N) by using the above formula. Thus WAP illustrates why CNRP is not a successful defeater to APM: on the assumption that T is true, there is no antecedent reason to expect CNRP over the denial of CNRP. In symbols, there is no reason to believe that Pr(~CNRP | T) is greater than Pr(~CNRP | T).

Objection: Physical embodiment provides a nice mechanism for the antithesis of certain omni-attributes that God might not want to endow creatures with – spatial location, lack of intrinsic eternality, limitations on knowledge, etc. It might be said that God could have simply created spirits or souls that lack the omni-attributes, but a) it seems easy to think of other good reasons for physical existence and on mind-body dualism the brain is the interface between the mind/soul and such existence, and b) since bringing about mind-brain dependence is at least *one* way of limiting the attributes of a created being, I’m not sure it matters whether or not there are other means of doing this.

Reply: I don’t deny that, if God exists, He might want to create embodied minds–indeed, for the very reasons mentioned by the objection. This is why (1) does not make the strong claim that E is antecedently much more probable on N than on T. Rather, (1) merely makes the weaker claim that E is antecedently more probable on N than on T. The fact that God might want to create embodied minds is not of obvious relevance to (1), however, since (1) does not claim (or entail the claim) that Pr(E | B & T) < 0.5. Rather, (1) makes a comparative claim, i.e., it compares the probability of E on naturalism to the probability of E on T. For the sake of argument, we could say that Pr(E | B & T) = 0.95. The fact remains that Pr(E | B & N) =1 whereas Pr(E | B & T) < 1. That’s all that is needed to defend (1).

Objections to (2)

Objection: Lowder is using a computer to communicate with the outside world. He’s also using a compute to learn about the outside world. Conversely, his computer allows other people to contact Lowder. So it’s a two-way street. If Lowder’s modem were damaged, he’d be sealed off from the world in that respect. But that would just mean the medium or conduit was blocked. Therefore, E1.2 is false.

Reply: If human minds were independent of the physical brain, then brain injuries should not have much, if any, impact on mental activity since, ex hypothesi, mental activity does not occur in the brain to begin with. Thus, E1.2 is antecedently much more probable on the assumption that E is true than on the assumption that E is false, i.e., Pr(E1.2 | E) >! Pr(E1.2 | ~E).

The objector’s computer analogy does not explain E1.2. If my computer were stolen, no amount of fiddling with it would enable the burglar to affect my ability to speak, recognize faces, feel pain. In contrast, brain injuries can produce exactly these kinds of results.

Objection: Regarding E1.4, is it really that simple or direct? Let’s take a few examples. In my observation, little dogs can be smarter than dogs 10, 15, 20 times their size. The bigger dogs have bigger brains, yet they’re dumber than the smaller dogs. Do the smaller dogs have more complex brains?

Reply: Regarding E1.4, when we compare different classes within the animal kingdom (e.g., fish, amphibians, reptiles, birds, mammals), we find that more intelligent classes always have more complex brains. I am not a neuroscientist or biologist, but I suspect that there is some margin of error with the correlation between brain size and mental capacities, e.g., an animal with a 500cc brain is always going to be more intelligent than an animal with a 5cc brain, but there may be instances within the same species of an animal with a 450cc brain that is more intelligent than an animal with a 500cc brain.

Objection: Regarding E.1.4, Social insects do very ingenuous things. If social insects were the size of chimpanzees (a scary thought!), it would be tempting to attribute their ingenuity to their brainpower.

Reply: Again, I am not a biologist, but I doubt that any biologist thinks that the intelligence of social insects is even in the same league as that of chimpanzees.

Objection: Likewise, predatory insects mimic the behavior of predatory mammals. Stalking their prey. Or ambush predators. When wolves, lions, leopards, and Cape hunting dogs do this sort of thing, it’s tempting to chalk that up to their mammalian brainpower.

Reply: Yes, but (I’m told) predatory insects are much more limited in their behavioral responses than social mammals like wolves, lions, leopards, and dogs. The latter are much more versatile in their behavioral responses because of their greater complexity. I could be wrong, but I believe this point is uncontroversial among entomologists and ethologists.

Objection: I don’t deny that souls and brains affect each other in subtle, intricate ways. But it seems to me that Lowder is appealing to half-truths. Ignoring counterevidence and oversimplifying the interrelationship.

Reply: Given that my argument is an evidential argument, this is false. Here considerations like the explanatory virtues come into play. What is the most parsimonious, scientifically conservative, successfully predictive explanation that accounts for the widest range of facts? Judged like any other empirical hypothesis, E is the best explanation, hands down.

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