Six days after The Financial Times launched an attack on the data behind Thomas Piketty’s much-debated tome on inequality, “Capital in the Twenty-First Century,” Mr. Piketty has offered his first detailed response to the newspaper’s criticism.

The short version: He doesn’t give an inch.

In response to a request from The New York Times to further address the criticisms, which The Financial Times published on Friday, Mr. Piketty, a professor at the Paris School of Economics, wrote that his data were correct, and his conclusions stood: Wealth inequality in Europe and the United States was high in the years before World War I, fell for much of the 20th century, and has been rising sharply again in the past three decades.