Obama has mentioned “the arc of history” a dozen times since his election.

The problem with this kind of thinking is that it imputes an agency to history that doesn’t exist. Worse, it assumes that progress is unidirectional. But history is not a moral force in and of itself, and it has no set course. Presuming otherwise embraces the dangerous tendency that the great English historian Herbert Butterfield dissected in his 1931 essay, The Whig Interpretation of History.