The universe appears to behave in logical ways. All of the individual physical components of the universe, as far as we can tell, are likely governed by logically consistent laws of physics. According to physicalism, human beings are nothing more than complex physical systems. That means that the physical components and functions of a human being, including those that give rise to human thought, are governed by the same logically consistent laws that govern the behavior of electrons, etc. If the physical processes that give rise to thought are rational how can a human being have an irrational thought? Where in the system does irrationality arise? It seems that human beings are in fact capable of irrational thought. If two people hold mutually exclusive ideas then at least one of them must be wrong. But if irrational thought is possible where does it come from? Is this an argument against physicalism? Does it mean we are more than just bits of matter? Or does it mean that the universe itself doesn't follow the rules of logic in all cases? Or is my reasoning faulty? If so, how is that possible according to the physicalist? p.s. I've just been listening to some philosophy lectures online and have nobody else to ask about this.

You asked, "If the physical processes that give rise to thought are rational, how can a human being have an irrational thought?" You might be misinterpreting the claim that "the physical processes...are rational." Presumably what's meant by the claim is that the physical processes can be discovered and understood by rational means, such as empirical investigation and logical reasoning. The claim doesn't mean to attribute rationality to the physical processes themselves: the processes don't literally investigate or reason, either well or badly. So the fact that the physical processes can be discovered and understood rationally doesn't imply that irrational thoughts can't result from those processes. Furthermore, we can rationally investigate the physical causes of irrational thoughts, even if science isn't very far down that road at present. In any case, we should resist the suggestion that the universe sometimes violates the laws of logic: that suggestion is either impossible or not even intelligible.

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