If his mental state is invisible to us, that means we can’t prove what his mental state is, but it should still be possible to have evidence for his mental state and to know it to some degree of certainty that isn’t 100%. Which is no different from what science does to “prove” anything else.
It would be difficult to say what this evidence would be. As one who has spent some time with people who would generally be called deluded, I can assure you that finding an understandable explanation for their delusions is non-trivial.
If his mental state is invisible to us, that means we can’t prove what his mental state is, but it should still be possible to have evidence for his mental state and to know it to some degree of certainty that isn’t 100%. Which is no different from what science does to “prove” anything else.
It would be difficult to say what this evidence would be. As one who has spent some time with people who would generally be called deluded, I can assure you that finding an understandable explanation for their delusions is non-trivial.