Consider the following argument against materialism (the thesis that only physical things exist):

1. If materialism is true, then the deliverances of our cognitive faculties are caused by a purely physical system (eg. our brain)
2. Purely physical systems are not purposive
3. Rationality is purposive
4. Therefore, if materialism is true, then we are incapable of being rational
5. Therefore, if materialism is true, we cannot rationally assent to materialism
6. 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.