Does capitalism have a natural tendency towards a just and reasonable distribution of wealth? The French economist Thomas Piketty thinks not.

In his bestselling 2013 book, Capital in the Twenty-First Century, Piketty takes issue with the idea that—despite the odd bump along the way, not least the 2007–08 global financial crisis—inequality (the gaps in income and wealth between rich and poor) tends to decline as capitalism matures.

Piketty spent 15 years building an unparalleled database on wealth and income in France, the United States, and a number of other countries. He uses this data to argue that the opposite is true. Capitalism’s natural tendency is, he says, to move toward ever-greater inequality. Piketty’s solution? A global wealth tax—even if he admits it has little chance of becoming a reality.

Capital has attracted impassioned responses, both positive and negative. But it has single-handedly shifted the goalposts of economic thinking and re-kindled discussion of the problem of inequality.