My friend Roderick Long has posted an interesting item on aesthetic criticism. His thrust is that critics generally say that a work of art should only be criticized on purely aesthetic grounds, but in actually doing reviews, they bring in other criteria, such as the artist's insight into human life.
Why do critics pay lip service to this "Neutrality Rule," but violate it in practice? Roderick speculates that the reason is that critics, realizing that something like "insight into life" by itself cannot make for aesthetic value, then mistakenly conclude that it plays no part in determining aesthetic value, because they naively view the aesthetic value of an entire work as being a merely additive function of the value of the various elements that went into it.
However, I see three other possible explanations:
1) Critics are correct in believing that an aesthetic judgment of a work of art proceeds on purely aesthetic grounds, but rarely live up to such a principle in practice. Similarly, one might hold, along with Collingwood, Mises, Oakeshott, and others, that the ideal character of history excludes historians' political views, while acknowledging that actual historians rarely if ever achieve that ideal.
2) What appears to be "insight into human nature" in a work of art is an artistic illusion, much as the "depth" in a Renaissance painting is illusory.
3) Things such as "insight into human nature" are important to works of art only in that they contribute to its overall aesthetic impact, rather than being important in and of themselves.
I believe the "Neutrality Rule" is sound, and that a combination of the three reasons I list above explains why it is often apparently or actually violated. Of course, the third reason I list is not entirely at odds with Roderick's explanation.