When one believes something on the basis of certain reasons, one takes those reasons to explain why one’s belief is true. For example, if I believe that Socrates is a mortal because all men are mortal and Socrates is a man, I do not simply think that my belief is causally explained by those reasons. Rather, I also take those reasons, insofar as they are the causes of my belief, to explain why my belief that Socrates is a mortal is true. If I did not take those reasons to explain why my belief that Socrates is a mortal is true, I would not believe that Socrates is a mortal at all.
If the same reasons that cause my beliefs also explain why my beliefs are true, then the content of my beliefs cannot depend on anything other than the character of the reasons that cause them. If they depended on anything further, then the fact that certain reasons caused my beliefs would not also be sufficient to explain the truth of those beliefs. Instead, an explanation of the truth of my beliefs would depend upon the nature of the further factor that influenced the content of my beliefs in addition to the reasons that caused me to form them. This would prevent me from from ever recognizing my beliefs as true by recognizing my reasons for forming them.
If mind-body physicalism is true, then our beliefs are physical phenomena that are governed by the laws of nature and that stand in causal relations with other physical phenomena. These laws determine the causal relations that obtain between our beliefs and the causes that explain them. Furthermore, these laws are not sensitive to rational relations of implication or entailment. If this is the case, then the truth of our beliefs does not simply depend on the causes of those beliefs. Rather, it also depends on the natural laws of our universe. Specifically, it depends on whether the natural laws of our universe entail causal relations that ensure the truth of beliefs that are caused by the appropriate reasons. This is precisely the kind of further factor that prevents a subject from ever recognizing their beliefs as true on the basis of their reasons for forming them. In order to know whether my beliefs were true, in this case, I would first have to determine whether the laws of nature were of the appropriate sort. But any belief about the laws of nature would itself have to be recognized as true on the basis of the reasons that explained it. This is impossible unless I already take the contents of my beliefs to be explained solely in terms of the character of the reasons that cause them. Consequently, a belief in mind-body physicalism precludes the rational possibility of recognizing the truth of our beliefs, including our belief in mind-body physicalism.
In short, if the reasons that cause our beliefs also explain their truth, then the contents of our beliefs do not depend on anything other than the character of the reasons that cause them. But if mind-body physicalism is true, then the contents of our beliefs do depend on something other than the character of the reasons that cause them. So if mind-body physicalism is true, then it is not the case that the reasons that cause our beliefs also explain their truth. The truth of this last consequent is incompatible with the possibility of rational warrant for one’s beliefs. Therefore, mind-body physicalism is incompatible with rational warrant for one’s beliefs, including one’s belief in mind-body physicalism.
Addendum:
While the argument above is an objection to mind-body physicalism alone, related (though more controversial) considerations will also apply to anti-physicalist alternatives.
If the mind is a physical or a non-physical substance, then there will necessarily be some truths that it cannot know. In particular, it will be incapable of knowing its own nature precisely because the truth of any belief that a mind has about its own nature will depend upon that nature itself. In order to know that the reasons that cause my beliefs also explain their truth, I must have some understanding of the nature of belief and the mind. In particular, I must know that beliefs are such as to be caused by reasons that ensure their truth. But this knowledge is precisely what is unavailable to me insofar as the mind is a physical or non-physical substance. I have no basis for trusting any belief that I form about the nature of a mental substance because the recognition of any trustworthy basis would depend upon knowledge of the very mental nature that I am trying to grasp. If I believe that a mental substance normally forms true beliefs, I can only trust this judgment insofar as I know that my beliefs are responsive to reality. But this is precisely what I need to establish in the first place.
I can only know the nature of my mind insofar as I know that this nature is simply what i think it to be. Insofar as it is anything other than what I think it to be, its nature lies beyond me and I cannot know that beliefs are such as to be responsive to objective reality. I do not know the mind to be a physical or a non-physical substance in thought. I do not know it to be any substance at all. Therefore, if the mind is a physical or a non-physical substance, I cannot know its nature and I cannot know my beliefs to be warranted. But I do know my beliefs to be warranted because I know that the mind is no physical or non-physical substance.
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