“I have a dream that my four little children will one day live in a nation where they will not be judged by the color of their skin but by the content of their character.”

This sentence spoken by the Rev. Martin Luther King Jr. has been quoted countless times as expressing one of America’s bedrock values, its language almost sounding like a constitutional amendment on equality.

Yet today, 50 years after King shared this vision during his most famous speech, there is considerable disagreement over what it means.