Thomas Piketty likes capitalism because it efficiently allocates resources. But he does not like how it allocates income. There is, he thinks, a moral illegitimacy to virtually any accumulation of wealth, and it is a matter of justice that such inequality be eradicated in our economy. The way to do this is to eliminate high incomes and to reduce existing wealth through taxation.

"Capital in the Twenty-First Century" is Mr. Piketty's dense exploration of the history of wages and wealth over the past three centuries....