Evolutionary argument against naturalism

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The evolutionary argument against naturalism (EAAN) is a philosophical argument regarding a perceived tension between biological evolutionary theory and philosophical naturalism --- the belief that there are no supernatural entities or processes. EAAN argues that the combination of evolutionary theory and naturalism is self-defeating on the basis of the claim that if both evolution and naturalism are true, then—according to Plantinga's calculations—the probability of having reliable cognitive facilities is low. The argument was proposed by Alvin Plantinga in 1993 and "raises issues of interest to epistemologists, philosophers of mind, evolutionary biologists, and philosophers of religion".


Plantinga's 1993 formulation of the argument

Plantinga's argument attempted to show that to combine naturalism and evolution is self-defeating, because, under these assumptions, the probability that humans have reliable cognitive faculties is low or inscrutable. He claimed that several thinkers, including C. S. Lewis, had seen that evolutionary naturalism seemed to lead to a deep and pervasive skepticism and to the conclusion that our unreliable cognitive or belief-producing faculties cannot be trusted to produce more true beliefs than false beliefs. He claimed that "Darwin himself had worries along these lines" and quoted from an 1881 letter:

But then with me the horrid doubt always arises whether the convictions of man's mind, which has been developed from the mind of the lower animals, are of any value or at all trustworthy. Would any one trust in the convictions of a monkey's mind, if there are any convictions in such a mind?

– Charles Darwin, to William Graham 3 July 1881

In the letter, Darwin had expressed agreement with William Graham's claim that natural laws implied purpose and the belief that the universe was "not the result of chance", but again showed his doubts about such beliefs and left the matter as insoluble. Darwin only had this doubt about questions beyond the scope of science, and thought science was well within the scope of an evolved mind. Michael Ruse said that by presenting it as "Darwin's doubt" that evolutionary naturalism is self-defeating, Plantinga failed to note that Darwin at once excused himself from philosophical matters he did not feel competent to consider. Others, such as Evan Fales, agreed that this citation allowed Plantinga to call the source of the problem EAAN addresses Darwin's Doubt. Also, contrary to Ruse's claim, Plantinga didn't label the idea that naturalism & evolution are self-defeating "Darwins Doubt". Instead he labeled the idea that given evolution and naturalism our cognitive faculties may not be reliable "Darwins Doubt". Plantinga asserts that "this doubt arises for naturalists or atheists, but not for those who believe in God. That is because if God has created us in his image, then even if he fashioned us by some evolutionary means, he would presumably want us to resemble him in being able to know; but then most of what we believe might be true even if our minds have developed from those of the lower animals."

Plantinga defined:

N as naturalism, which he defined as "the idea that there is no such person as God or anything like God; we might think of it as high-octane atheism or perhaps atheism-plus." E as the belief that human beings have evolved in conformity with current evolutionary theory R as the proposition that our faculties are "reliable", where, roughly, a cognitive faculty is "reliable" if the great bulk of its deliverances are true. He specifically cited the example of a thermometer stuck at 72 °F (22 °C) degrees placed in an environment which happened to be at 72 °F as an example of something that is not "reliable" in this sense and suggested that the conditional probability of R given N and E, or P(R|N&E), is low.

Plantinga's argument began with the observation that our beliefs can only have evolutionary consequences if they affect behaviour. To put this another way, natural selection does not directly select for true beliefs, but rather for advantageous behaviours. Plantinga distinguished the various theories of mind-body interaction into four jointly exhaustive categories:

1.epiphenomenalism, where behaviour is not caused by beliefs. "if this way of thinking is right, beliefs would be invisible to evolution" so P(R/N&E) would be low or inscrutable

2.Semantic epiphenomenalism, where beliefs have a causative link to behaviour but not by virtue of their semantic content. Under this theory, a belief would be some form of long-term neuronal event. However, on this view P(R|N&E) would be low because the semantic content of beliefs would be invisible to natural selection, and it is semantic content that determines truth-value.

3.Beliefs are causally efficacious with respect to behaviour, but maladaptive, in which case P(R|N&E) would be low, as R would be selected against.

4.Beliefs are causally efficacious with respect to behaviour and also adaptive, but they may still be false. Since behaviour is caused by both belief and desire, and desire can lead to false belief, natural selection would have no reason for selecting true but non-adaptive beliefs over false but adaptive beliefs. Thus P(R|N&E) in this case would also be low. Plantinga pointed out that innumerable belief-desire pairs could account for a given behaviour; for example, that of a prehistoric hominid fleeing a tiger: Perhaps Paul very much likes the idea of being eaten, but when he sees a tiger, always runs off looking for a better prospect, because he thinks it unlikely the tiger he sees will eat him. This will get his body parts in the right place so far as survival is concerned, without involving much by way of true belief. ... Or perhaps he thinks the tiger is a large, friendly, cuddly pussycat and wants to pet it; but he also believes that the best way to pet it is to run away from it. ... Clearly there are any number of belief-cum-desire systems that equally fit a given bit of behaviour.

Thus, Plantinga argued, the probability that our minds are reliable under a conjunction of philosophical naturalism and evolution is low or inscrutable. Therefore, to assert that naturalistic evolution is true also asserts that one has a low or unknown probability of being right. This, Plantinga argued, epistemically defeats the belief that naturalistic evolution is true and that ascribing truth to naturalism and evolution is internally dubious or inconsistent.


Plantinga's 2008 formulation of the argument

In the 2008 publication Knowledge of God Plantinga presented a formulation of the argument that solely focused on semantic epiphenomenalism instead of the former four jointly exhaustive categories.

Plantinga stated that from a materialist's point of view a belief will be a neuronal event. In this conception a belief will have two different sorts of properties:

electro-chemical or neurophysiological properties (NP properties for short)

and the property of having content (It will have to be the belief that p, for some proposition p).

Plantinga thought that we have something of an idea as to the history of NP properties: structures with these properties have come to exist by small increments, each increment such that it has proved to be useful in the struggle for survival. But he then asked how the content property of a belief came about: "How does it [the content] get to be associated in that way with a given proposition?"

He said that materialists offer two theories for this question: According to the first, content supervenes upon NP properties; according to the second, content is reducible to NP properties. (He noted that if content properties are reducible to NP properties, then they also supervene upon them.) He explained the two theories as follows:

Reducibility: A belief is a disjunction of conjunctions of NP properties.

Strong Supervenience (S+): For any possible worlds W and W* and any structures S and S*, if S has the same NP properties in W as S* has in W*, then S has the same content in W as S* has in W*. Supervenience can either be broadly logical supervenience or nomic supervenience.

Plantinga argued that neural structures that constitute beliefs have content, in the following way: "At a certain level of complexity, these neural structures start to display content. Perhaps this starts gradually and early on (possibly C. elegans [a small worm with a nervous system composed of only a few neurons] displays just the merest glimmer of consciousness and the merest glimmer of content), or perhaps later and more abruptly; that doesn't matter. What does matter is that at a certain level of complexity of neural structures, content appears. This is true whether content properties are reducible to NP properties or supervene on them." So given materialism some neural structures at a given level of complexity acquire content and become beliefs. The question then is according to Plantinga: "what is the likelihood, given materialism, that the content that thus arises is in fact true?"

This way of proceeding replaced the first step of Plantinga's earlier versions of the argument.

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