I don't think we are all irrational every time we fail to see through an argument in a book, but suppose it's true about you. You are still more rational than you think you are. You are irrational in a minor way - believing a misguided theory of the nature of rationality - but rational in a major way - you respond well to probabilistic evidence as you go through the day.

Though we think intrinsic desires tend to be pretty stable, we do not think they imply anything like the amount of predictability in behavior that traditional virtue ethics requires for someone to have a one-word-in-English character trait such as "benevolence". Other things being equal, a person with more of a desire for other people's wellbeing will do more for other people's wellbeing, but things are almost never equal.

Suppose a student of mine writes in her exam that "morality is completely relative to culture, so nothing is absolutely right or wrong. Because of that, it is absolutely wrong to be culturally intolerant". This student, if she believes what she writes, believes a contradiction. She ought not to believe the contradiction - it's a basic epistemic norm. This is true even if she can't avoid believing it - no amount of studying will show her the light.

I am not committed in any way to the traditional concept of character - the concept of "character trait" as involving predictable behavior. I am committed to a view in the neighborhood - the view that the moral worth of one's actions depends on the quality of will expressed in them.

Virtue is about wanting right and good things, not about being particularly good at thinking.

Sometimes I suspect that there are two prototypes of philosophers who write about humans - I call them "celestials" and "terrestrials", without implying that celestials have their heads in the clouds or that terrestrials have theirs buried in the ground. The difference between these two types is not so much in their theories but in whether or not they would find it a very sad thing if it turned out that the only way a human is superior to a wolf is this: the human brain is significantly more capacious and complex.

I think it's ok to have wishes that conflict with each other - it's irrational to try to make them both come true, but not irrational simply to have them.