|Evolutionary Argument Against Naturalism|
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The Probability Thesis
Why think Pr(R|N&E) is low? Ordinarily one might think that true beliefs help us survive. That certainly is the case if the content of our beliefs is causally relevant to behavior (e.g. I believe this plant is poisonous so I won’t eat it). But if the truth of our beliefs has no such causal relevance, then such a factor will be invisible to natural selection in the sense that while natural selection selects for adaptive physiologies and certain physiologies produce semantic content, which semantic content gets generated from a given physiology doesn’t affect natural selection’s outcome. So the content of our beliefs could be anything, true or not, and it wouldn’t affect our behavior. Whether the belief's content is 2 + 2 = 4, 2 + 2 = 67, or 2 + 2 = 4096 would make no difference to how we behave. If that is true, then Pr(R|N&E) is low.
The sort of naturalism being discussed here assumes that we human beings are purely physical creatures with no nonphysical minds or souls. In my article Plantinga’s Argument against Materialism I described Alvin Plantinga’s argument for the idea that if materialism with respect to human beings were true (i.e. if we were purely physical beings), the propositional content of our beliefs (e.g. There is a cold soda in the fridge) would not be causally relevant. For convenience, I’ll recapitulate some of that here.
To illustrate the idea of a belief’s semantic content being causally relevant to behavior, suppose I want a cold soda, and my roommate informs me that there is cold soda in the fridge. The belief There is a cold soda in the fridge is (part of) what causes me to go to the fridge and get a cold soda. We would naturally think it is by virtue of the belief’s content that this belief causes me to go over to the fridge. On dualism (the view that our minds are a composite of the physical brain and a nonphysical mental component, e.g. the soul) it is possible for a belief’s content to affect behavior; e.g. I believe something and on the basis of this belief my soul impacts my neural pathways in a certain way to cause behavior.
But on materialism, the coin of belief has two sides: the neurophysiological (NP) properties (certain neurons being connected in a certain way etc.) of the belief, and the actual semantic content of the belief (e.g. There is a cold soda in the fridge). If materialism were true, the content of a belief is causally irrelevant in the sense that (given materialism) a belief causes stuff by virtue of its NP properties, and not by virtue of its content. We can see this by doing a little thought experiment. Suppose a given person’s belief (a neural structure possessing semantic content), say, the belief that There is a cold soda in the fridge, had the same NP properties but an entirely different content—such as The moon is made of green cheese. Would the person’s behavior be any different if the belief had the same NP properties but different content? It would not, because 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. Thus if materialism were true, the content of our beliefs would be causally irrelevant. The view that a belief causes stuff by virtue of its NP properties and not its semantic content is called semantic epiphenomenalism (SE).
In response the critic of SE could say that there’s something wrong with the thought experiment because it is metaphysically impossible for a given set of neurophysiological properties to have a different semantic content. But even if it is impossible for a given set of NP properties to have a different semantic content associated with it, does this prevent the statement “If a belief had the same NP properties but different content, the same behavior would result” from being meaningfully true? I think not. In philosophy, statements of the form, “If P were true, then Q would be true” where P is an impossibility are called counterpossibles. It does seem that there are counterpossibles that are meaningfully true. For example, suppose renowned mathematician Kurt Gödel proved a certain theorem; it is impossible for theorems to be proved false since they are necessarily true. Yet as Alvin Plantinga points out, “If Mic were to prove Gödel wrong, mathematicians everywhere would be astonished; it is not true that if Mic were to prove Gödel wrong, mathematicians everywhere would yawn in boredom.” So even if “If a belief had the same NP properties but different content, the same behavior would result” were a counterpossible, this doesn’t seem to prevent the statement from being meaningfully true.
More could be said about SE. If you’re still not convinced, I argue for it a little more in the reductive and nonreductive materialism section in my article on Plantinga’s argument against materialism (don’t worry, I explain what both sorts of materialism are!).
So why would it matter if N&E entails that the semantic content of our beliefs is causally irrelevant? To avoid bias against our own species, think not of us but of alien creatures whose physiology is radically unlike our own, and let RA represent the cognitive faculties of the aliens are reliable. N&E is true for these aliens, thus making the semantic content of their beliefs causally irrelevant. Then on N&E the electrochemical reactions that cause the behavior of these 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. To illustrate the potential problem this creates, suppose a random belief is assigned for the answer to What does two plus two equal? Answers of one, two, rock, and sunshine would all be wrong. Randomly selected beliefs about the color of the sky and one’s age are similarly likely to be wrong. The enormous variety of “garbage” belief sets akin to dreams vastly outnumber those belief sets that accurately resemble one’s external reality. Since which belief set gets produced is invisible to natural selection, while it is still possible for the electrochemical reactions that produce advantageous behavior to also produce a reliably true belief set (as opposed to a garbage, dream-like one), it would be the most serendipitous of coincidences if that were to occur. Thus in the absence of further relevant information, the likelihood that their cognitive faculties are reliable (given N&E) is low, and thus Pr(RA|N&E) is low.
One could object saying that while the probability of cognitive reliability is low given just N&E, we know of further relevant information P such Pr(R|N&E&P) is high, e.g. we know that for own physiology, the link between content and behavior is favorable for our species, such that we act as if semantic content influences our behavior in a manner befitting a rational agent. Maybe that’s true, but that’s an objection to the Defeater Thesis and not the Probability Thesis. For now we’re just concerned with justifying Pr(R|N&E) being low. I’ve argued that Pr(RA|N&E) is true for 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.
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 Plantinga, Alvin. “A New Argument against Materialism” Philosophia Christi 14.1 (Summer 2012) p. 21