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.
Nomy ArpalyVirtue is about wanting right and good things, not about being particularly good at thinking.
Nomy ArpalyA person is praiseworthy for a right action to the extent that her action manifests, and is rationalized by, good will, that is, concern for the right and the good, not necessarily under the description "right" or "good". A person is blameworthy for a wrong action to the extent that her action manifests, and is rationalized by, ill will - concern for the wrong and bad, also de re - or moral indifference - lack or deficiency of good will.
Nomy ArpalyThough 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.
Nomy ArpalyIf I make a stupid decision but don't execute it because I'm, say, lazy, then I'm lucky, not rational. However, at other times a person acts for good reasons just as she does what she thinks she shouldn't do, not knowing that they are good reasons. Just like sometimes we are a lot less rational than we think we are, it is also true that sometimes we are a lot more rational than we think we are.
Nomy Arpaly