Let us examine the following familiar syllogism: 1) All men are mortal, 2) Socrates is a man, 3) Therefore, Socrates is mortal. This syllogism is logically sound, and it can be reasonably seen why the conclusion follows from the premises. But, how exactly is it the case that we reasonably deduce the conclusion? Well, it’s the case because we understand the semantics behind every proposition in the syllogism. That is, we clearly comprehend what 1) and 2) mean, and we logically deduce the third proposition (the conclusion) based on the meaning of the preceding premises. This is the process of reasoning, namely, making deductions based on the semantic content of previous concepts, propositions or ideas. Thus stated, our deduction from 1) and 2) to 3) is based on meaning, semantics, and reason.
Now what is going on psychologically when we make our way, logically, through this line of reasoning? Well, we have a brain state and a mental state associated with upholding premise 1), and the same with regards to 2) and 3). The brain state is just what is happening electrochemically in my brain when I uphold a certain proposition or thought, and the mental state is the associated with the actual mental apprehension of said proposition or thought. (Note: even if one is an eliminative materialist and therefore doesn’t believe mental states exist, the distinction between brain and mental states is not crucial to my present argument, and would actually make the force of the argument stronger.) It should be obvious that brain states and mental states are different with regards to the different propositions we are upholding. That is to say, the brain state and mental state we have for 1), will be different from the brain and mental state we have for 2), and 3).
However, one immediately encounters a problem with the above promulgations if one is a naturalist. Why? Well, remember that a naturalist believes that all that exists is mindless matter, or energy, governed by the laws of physics. A corollary of this is that any thoughts (and, therefore, any mental states) are ultimately the result of physical processes swinging to the tune of physics. But, this means that the mental states associated with upholding 1), 2) and 3) are ultimately grounded in, and caused by, the electrochemical properties of brain states. A consequence of this is that when the brain state associated with 3) succeeds 2), which succeeds 1), this succession was the result of only physical laws acting on the matter that makes up these brain states.
But, this completely conflicts with what we expounded above, namely, that is it the meaning and semantics associated with our syllogism that causes us to arrive at such a deduction. On the naturalistic account of the world, when one makes their way down the syllogism, it is not the meaning or semantics that leads one there, but, rather, purely mindless electrochemical processes. But this means that, on naturalism, it is not reason-- making deductions based on the semantic content of previous concepts, propositions or ideas—that grounds our thinking. In fact, based on what we’ve discovered, there is no room for such a thing as reason on naturalism!
The naturalist position, then, results in a reductio ad absurdum. That is, if naturalism were true, then we could not reason, which means we could not arrive at the conclusion of naturalism, since to do so would require reasoning deductively. Naturalism, therefore, cannot possibly be true.