There has been an increase in inequality within labor income, Mr. Furman argues. At the top end, superstars in fields like finance and sports are paid more than ever, both in absolute terms and relative to typical workers. Meanwhile, wages have been depressed at the middle and lower income brackets by such factors as technological change, a decline in unionization and a decline in the inflation-adjusted minimum wage.

Image Jason Furman, chairman of the Council of Economic Advisers, has questioned some assumptions in a book by the French economist Thomas Piketty. Credit... T.J. Kirkpatrick/Getty Images

But while those changes are real and important, particularly in the United States, the crux of Mr. Piketty’s argument is that the more important shift over the long run is toward enormous accumulations of wealth, in the form of capital, which, so long as returns on capital rise more quickly than the overall economy and taxes on capital remain low, can create a vicious cycle of ever-growing dynastic wealth. This is what happened, he argues, in the 19th century, a cycle broken only by the wars and political revolution of the first half of the 20th century.

Or as Mr. Furman summarizes it in the prepared text of his speech: “The most striking argument in Piketty’s book is that to the degree that growth rates slow in the future because of demographic or other factors, this will inevitably lead to a sustained increase in inequality.”

But Mr. Furman isn’t completely buying it. “This thesis is intriguing and an important source of concern,” he said, “although it is unclear how likely it is. Piketty’s prediction is that the capital share of income will rise, pushing in the direction of increased inequality. But this is only one of the determinants of inequality. While the trends may continue to shift in that direction, a more important factor to date has been the inequality within labor income, and while Piketty implicitly takes this to be fixed, there is no a priori basis to predict whether it will rise or fall in the future because it is a function of unpredictable technological developments, norms, institutions and public policies.”

In other words, the starting point of Mr. Piketty’s analysis is the trends that lead to superstars in a range of industries earning millions or billions of dollars, which they can in turn invest in capital that builds on itself. But there are no guarantees that the trend of recent decades in this direction will continue.