Near the end of Richard Weaver's essay, "Life Without Prejudice," he quotes Milton:
I cannot praise a fugitive and cloistered virtue unexercised and unbreathed, that never sallies out and sees her adversary, but slinks out of the race, where that immortal garland is to be run for, not without dust and heat. Assuredly we bring not innocence into the world; we bring impurity much rather; that which purifies us is trial, and trial is by that which is contrary.
This fits nicely with Keat's notion of the world as a vale of soul-making.