Consider the following argument against materialism (the thesis that only physical things exist):
- If materialism is true, then the deliverances of our cognitive faculties are caused by a purely physical system (eg. our brain)
- Purely physical systems are not purposive
- Rationality is purposive
- Therefore, if materialism is true, then we are incapable of being rational
- Therefore, if materialism is true, we cannot rationally assent to materialism
- Therefore, materialism is self-defeating
(1) seems plausible, since on materialism only physical systems exist. When I say something is “purposive” I mean that it seeks to achieve some goal, ie. there’s teleology. So (2) seems plausible, since physical systems are typically understood to behave based on previous conditions, not in an attempt to bring about an outcome. Why think (3) is true? Well, if someone comes to a conclusion, but they weren’t trying to be rational in their reasoning (ie. seeking the rational conclusion) or they weren’t seeking the truth, then can we really say they were being rational? I don’t think so.
Thinking Thought Out 
Reminds me of Feser’s AfR. Basically, given materialism, the material processes of the brain (or any physical system) have their causal power regardless of any meaning that is assigned to them. The brain states that correspond to “All men are mortal” and “Socrates was a man” will lead to the next brain state only by virtue of physical properties, not the premises that are “attached.” A physical object does not gain or lose any physical properties if meaning is assigned to it. Under materialism It follows that when a materialist is “reasoning” to some conclusion, it is not the premises/meaning that lead one thought to another, but just physical properties.