Here are a few reasons I believe naturalism undermines rationality and ends up being self-defeating under the claim that the natural world/matter is all there is:
To begin with, naturalism/materialism is one of the most primitive philosophies there is and was popular with the pre-Socratic philosophers until refuted by Socrates, Plato and Aristotle. It has only gained resurgence to the extent modern people have forgotten these great philosophers and their clear thinking on this issue.
The idea of the materialist is that matter is all there is and the universe is causally closed. Everything is to be explained solely in terms of physics, matter and motion.
Many things have been shown to be highly problematic for this view especially in regard to the philosophy of mind including consciousness, thoughts, intentionality, identity over time, etc.
In the next few paragraphs I wish to briefly show that a dogmatic assertion of naturalism is irrational, often self-referentially inconsistent, and false.
1. Naturalism is Irrational (to the extent one thinks it can be chosen on the basis of good reasons or evidence)- It should be obvious that naturalism is irrational when we consider how it reduces mind to merely the brain and thought to nothing but the processes of physics and chemistry.
Because if my mental processes are nothing more than the determined motions of atoms in my brain I have no good reasons to affirm that my beliefs are true. If one of my beliefs is that naturalism is true then I cannot choose to believe it on the basis of reason or evidence because evolution only selects for survival traits irrespective of whether they are true.
Arguing if favor of naturalistic determinism is irrational because in order to argue for your position you have to do so in a way that assumes your position is untrue and others have the freedom to assess the evidence and reason to the same conclusion you have.
Even if naturalism were true you could never KNOW it because we cannot trust our natural processes to give us truth because nature chooses not for truth but for survivability. Instead, all of the beliefs we hold are there simply because it is predetermined that they be what they are.
Without free will there is no rationality. Without rationality there is no knowledge. Ergo- If naturalism is true there is no knowledge.
Therefore, Naturalism is irrational.
2. Naturalism is Self-Referentially Inconsistent (to the extent the Naturalists says we can think Naturalism is true on the basis of reasons or evidence)- As a Naturalist you can reasonably say, “I believe in naturalism.” But if you go further and say, “Nature is all there is” or “I have good reasons for believing in naturalism” then your claim is self-refuting for all the reasons listed in the previous paragraph.
Naturalism simply does not give you the option to claim you have good reasons for embracing what you consider rational judgments. Having reasons for believing in naturalism is incompatible with naturalism if it is true and you are being self-referentially inconsistent every time you try to offer reasons for naturalism or try to change other people’s minds. You cannot offer rational reasons for something if people have no free will to accept it and if all our mind is is brain and natural processes we have no choice in the matter.
Therefore, Naturalism is self-referentially inconsistent.
3. Naturalism is False (to the degree to which we know we can make rational judgments)- Because it is intuitively obvious from first person introspection that we can make rational judgments and decisions. I can choose to raise my arm right now if I wish. This is much more obviously true than naturalism and we have no good reason to deny it.
So we can form this argument:
“If naturalism is true then we are not justified in forming our beliefs.
But we are justified in forming our beliefs.
Therefore, naturalism is not true.”
So it is easy to reduce naturalism to absurdity and on the defense to ground itself in good reasons it has to ignore itself to even formulate.
Therefore, Naturalism is clearly false.
BONUS POINT: Another major self-defeater for a dogmatic assertion of Naturalism is that as a philosophical statement it is logically incoherent to assert that Nature is all there is because that is all we have empirically observed.
In principle, it is impossible to show that Nature is all there is simply by the observations we have made with the tools we use to measure and observe nature.
Using the sciences and phenomenological observations we can use inductive reasoning to study the fundamental constants of the universe and all the other things in the world of sense experience but it is utterly intellectually incoherent to move from a statement like “Well, I have observed the physical world” to saying, “Therefore the physical world is all there is.”
It is as silly as saying, “Well, I have walked all through my house and I did not see any Ford trucks- therefore there are no Ford trucks in existence.” It isn’t that there is no such thing as Ford trucks- the problem is that you simply have not looked in any relevant area to find one.
According to all the varieties of classical theism God is immaterial. You cannot hope to find an immaterial Being by looking at material things. And not even in principle is it possible to conclude that the physical universe is all there is simply by looking around the physical universe.
The most the naturalist can coherently say is, “I believe that Nature is all there is, there may or may not be more than that.” Never could he say, “Nature is all there is.”
Only an agnostic naturalist can affirm his position but he must do so without logic and reason on pain of self-referential inconsistency; and the gnostic naturalist, the one who is sure of his position, is simply committing an intellectual error of the first magnitude and is neck deep in pure ideological hubris.