Condescending to the Past

I like what The Atlantic's Benjamin Schwartz says in this review about T.S. Eliot's letters:

"Because I find the condescension of posterity—through which we applaud ourselves by imposing our enlightened standards on a supposedly benighted past—to be a particularly unattractive reflex..."

It's a great thing to remember when we write about the past. If it's fiction, you can juxtapose someone who condescends against someone who doesn't to create a nice frission of ideas.