Seriousness as a literary criterion.

Let say that literary criticism's most important task is to distinguish serious work from what we might call the "upper-middlebrow". But what does serious mean when it comes to literary writing? I'm wondering about this while in the middle of reading Andrew O'Hagan's Personality (2003) which I think is about as good an example of an uppermiddlebrow literary novel as you can find of its period.