The general convention in fiction is to write in the past tense, e.g. "John walked into a bar." But the convention in literary criticism when writing about the action in a piece of fiction is to write in the present tense, e.g. "Midway through the novel, John walks into a bar."
I suppose this makes sense; from the perspective of the narrator of a piece of fiction, the action has already happened and the narrator is describing it retrospectively. But for the literary critic, it seems as though the action happens as you read it. Although by the time the critic writes, it's already happened, so perhaps it should be past tense. And from the perspective of the reader, the action of the book hasn't happened yet. A literary critic could, in defiance of all laws of grammar, write his descriptions in the future tense: "If you read this book, midway through John will walk into a bar." But that's just silly.
What's more silly, though, is the convention in historical writing. In history writing, everything is in the past tense, no matter what. This leads to some awkward and imprecise phrases. For example, suppose you wanted to summarize Oliver Wendell Holmes's Memorial Day Address. The convention in literary criticism would be to write:
"In his Memorial Day Address, Holmes argues that war is a force that gives life meaning."
This seems somewhat inappropriate, but if you've grown accustomed to reading literary criticism you're used to long-dead authors being drawn into the present tense. The historian would write the sentence this way:
"In his Memorial Day Address, Holmes argued that war was a force that gave life meaning."
Now Holmes has been put in his proper place in the past, but so, unfortunately, have his sentiments. Holmes wasn't making a historical argument; he was making a philosophical statement. He intended to say that war would give life meaning as much in 2006 as it did in 1884. Moreover, we can't tell, reading the statement, whether Holmes even meant that it gave life meaning in 1884; the sentence can be plausibly read as saying "War once gave life meaning, but it does not any longer." Unfortunately, this is the proper way to write the sentence in a history paper. Absolutely everything that isn't a quote goes in the past tense. I would argue that the best way to write the sentence would be:
"In his Memorial Day Address, Holmes argued that war is a force that gives life meaning."
This puts Holmes in the past, and indicates that Holmes intended no restrictions on his meaning with respect to time. Still, the sentence feels wrong when said out loud, thanks to the mixing of tenses.
This is probably why literary critics keep things in the present tense. It sounds better and avoids at least some confusion. And eventually you stop noticing how odd it is to read about ancient Greek authors writing in the present tense.
