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As I mentioned in
Plantinga’s Argument against Materialism, Plantinga argues that even if it is true that a belief is causally relevant by virtue of it having a certain content, it doesn’t follow that the semantic content is itself causally relevant. To illustrate, suppose Alvin throws a ball that has a mass of 0.2 kilograms and the ball hits a glass window, causing it to break. If the ball had been much lighter (say, the mass of a feather) it would not have broken the glass, so the ball breaks the window by virtue of (among other things) being 0.2 kilograms. Now suppose the property of
having a mass of 0.2 kilograms is Sam’s favorite property. Thus we have the following:
| | Having a mass of 0.2 kilograms | = |
Sam’s favorite property |
Since
having a mass of 0.2 kilograms is Sam’s favorite property (it isn’t his favorite property by definition, but reductive materialists similarly don’t [typically] believe the identity relationship between a belief and a Boolean combination of NP properties holds by definition), it follows then that a ball breaks the window by virtue of having Sam’s favorite property. And yet, this:
| | Having a mass of 0.2 kilograms is Sam’s favorite property |
doesn’t really seem to have anything to do with why the ball breaks the window, even though it is by virtue of having Sam’s favorite property that the ball breaks the window. Similarly, the fact that a set of NP properties is a belief doesn’t seem to have anything to do with why the set of NP properties causes behavior, even if it is true that a belief causes behavior by virtue of having a certain semantic content.
I think we can make this clearer by considering the following thought experiment. Suppose reductive materialism is true and a mad scientist inserts a belief interface device (BID) in Smith’s brain that acts as a new interface between Smith’s belief and behavior. For example, the mad scientist configures the BID so that when Smith believes
I am thirsty the NP properties of this belief electrochemically affect the BID and the BID subsequently causes Smith’s body to get a drink of water. The mad scientist can configure the BID at will so that any given belief can cause just about any behavior. For instance, the mad scientist configures the BID so that the NP properties of the belief
Drinking water will kill me and I don’t want to die trigger an electrochemical reaction that (thanks to the belief interface device) causes Smith’s body to get a drink of water. The mad scientist configures the BID again so that the NP properties of the belief
Grass is air cause Smith to eat coconut ice cream. All this would be possible on reductive materialism because a human’s belief causes stuff by virtue of its NP properties, not its semantic content, even if it were the case that beliefs are just combinations of NP properties. Thus even on reductive materialism the semantic content of a belief is in a very real sense causally irrelevant.
To recap what we have so far: given materialism, a human’s belief is a neural structure possessing semantic content, and the semantic content of a belief is causally irrelevant in the sense that a belief causes stuff by virtue of its NP properties, and not by its semantic content. If a belief had the same NP properties but different semantic content, the same behavior would result (since having the same neurophysiological properties means we would have the same electrical impulses travelling down the same neural pathways and thus issuing the same muscular contractions). Even if reductive materialism were true and beliefs are just Boolean combinations of NP properties, beliefs appear to cause behavior by virtue of their NP properties, not by their semantic content. The BID scenario in particular illustrates that even on reductive materialism, the semantic content of a belief can be quite unrelated to the person’s external environment when that belief causes behavior, e.g. the situation where the NP properties of the belief
Grass is air cause Smith to eat coconut ice cream.
To help guard against bias towards our own species, think not of us but of some alien creatures where N&E is true with respect to them. Let
RA be “the cognitive faculties of the aliens are reliable.” In such a situation, it’s really quite remarkable that N&E implies the causal irrelevance of a belief’s semantic content. The electrochemical reactions that cause the behavior of the aliens could generate any semantic content at all (e.g.
2 + 2 = 1 or
Grass is air) without that content affecting behavior. The semantic content could even be “garbage” beliefs unrelated to the external environment, as in dreams, and it still wouldn’t affect behavior. Of course, it would still be possible that the electrochemical reactions that produce advantageous behavior also generate mostly true beliefs, but it would seem to the most serendipitous of coincidences if that were to occur, given the causal irrelevance of a belief’s semantic content. Pr(RA|N&E) is thus clearly low.
But then the same sort of thing would hold true for our human species, and we have the following argument:
- If Pr(RA|N&E) is low, then Pr(R|N&E) is low.
- Pr(RA|N&E) is low.
- Therefore, Pr(R|N&E) is low.
In response one could put forth the following rebuttal. Even though naturalism unavoidably entails that a belief’s semantic content is causally irrelevant, the fitness-enhancing NP properties that are most likely to be selected by natural selection (say that a certain neurophysiology is
selectable just in case it’s likely to be selected by natural selection) happen to be those that are truth-conducive in the sense that they yield mostly true beliefs. The BID scenario is contrived and produces certain belief-behavior pairs that are unlikely to obtain in real human physiology; the real way beliefs interface with behavior doesn’t work that way. The most selectable and efficient way for neurophysiology to produce advantageous behavior also produces true beliefs. Thus, even though a belief’s content is causally irrelevant, luckily for us the physiological relation between semantic content and behavior is such that true beliefs usually obtain.
All that may be true, but as an objection against the Probability Thesis it falls short. Remember, we are not evaluating the probability of R given “the way things are” (and I would concede that given the way things are, our cognitive faculties are probably reliable) but rather given N&E. For reasons given, it does not seem likely that if N&E
were true that our cognitive faculties would be reliable. Also, a major problem is that even if a favorable physiological relation between beliefs and behavior were to obtain for our species in spite of the causal irrelevance of our semantic beliefs,
such a favorable relation does not appear to be knowable from N&E alone. To illustrate, consider a planet with aliens whose neurophysiology radically different from ours (though we don’t know much more about it). Given this, the BID scenario, and the causal irrelevance of a belief’s content, for all we know the most selectable and efficient fitness-enhancing alien neurophysiology available to natural selection has a physiological relation between beliefs and behavior that is wildly different from what human naturalists believes about themselves (think of the case where the NP properties of
Grass is air cause Smith to eat coconut ice cream). So there are possible worlds where the most selectable alien neurophysiology is such that the fitness-enhancing neurophysiology produces mostly false beliefs. Of course, there are also possible worlds where the most selectable alien neurophysiology produces mostly true beliefs. But there’s no way to establish on N&E alone that the truth-conducive neurophysiology is more selectable, in part because the alien neurophysiology is too mysterious and too radically different from our own. Given
just N&E (and how it doesn’t matter which semantic content gets produced from the behavior-causing electrochemical reactions), for all we know the behavior-causing neurophysiology yields “garbage” beliefs that have little to do with the environment, as in dreams.
One could concede that the probability of R given N&E is low but also claim we know some proposition P (perhaps that the physiological relation between beliefs and behavior happens to be benevolent for our species) such that Pr(R|N&E&P) is high, and we have excellent reason to believe that P is true. Therefore, Pr(R|N&E) being low does not defeat R for the evolutionary naturalist. This however would be an objection against the Defeater Thesis rather than the Probability Thesis, so it will not be discussed in this section. Can the Defeater Thesis withstand this objection? For that matter, why accept the Defeater Thesis in the first place?
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[1] Plantinga, Alvin. “A New Argument against Materialism”
Philosophia Christi 14.1 (Summer 2012) p. 21
