A second try at a recent post.
Consider this page’s section “1. An Assertion”. No rational person can believe that the contents of this section are entirely true.
First, note that the assertion in section 1 does not depend on evidence or context. Whether I rationally believe it or not must rise or fall on reason alone.
If I believe that section 1 is true then I also believe that belief in section 1 is inconsistent with rationality, that no rational person can accept its truth. I must clearly then not be rational, for what rational being can believe that which they know to be inconsistent with rationality?
Conversely, I might suppose that all rational people will of necessity believe something, anything about section 1 other than its truth. Maybe some think it is false, or a paradox, or utter nonsense. Maybe some are utterly confused and decide to ignore it, thinking it neither true nor false. There may even be some who see in it the mortal echo of a transcendent thought that lies above such paltry details as truth and falsehood. Whatever the details, all such beliefs are versions of that which section 1 is asserting; in other words, any such belief entails that section 1 is true.
Ergo, I must not be rational.