The groundbreaking status of the book was recognised by a recent long essay in the New Yorker in which Branko Milanovic, a former senior economist at the World Bank, was quoted as describing Piketty's volume as "one of the watershed books in economic thinking". In the same vein, a writer in the Economist reported that Piketty's work fundamentally rewrote 200 years of economic thinking on inequality. In short, the arguments have centred on two poles: the first is a tradition that begins with Karl Marx, who believed that capitalism would self-destruct in the endless pursuit of diminishing profit returns. At the opposite end of the spectrum is the work of Simon Kuznets, who won a Nobel prize in 1971 and who made the case that the inequality gap inevitably grows smaller as economies develop and become sophisticated.

Piketty says that neither of these arguments stand up to the evidence he has accumulated. More to the point, he demonstrates that there is no reason to believe that capitalism can ever solve the problem of inequality, which he insists is getting worse rather than better. From the banking crisis of 2008 to the Occupy movement of 2011, this much has been intuited by ordinary people. The singular significance of his book is that it proves "scientifically" that this intuition is correct. This is why his book has crossed over into the mainstream – it says what many people have already been thinking. ... So I asked him the most obvious question I could: what is the big idea behind this book?

"I began with a straightforward research problematic," he says in elegant French-accented English. "I began to wonder a few years ago where was the hard data behind all the theories about inequality, from Marx to David Ricardo (the 19th-century English economist and advocate of free trade) and more contemporary thinkers. I started with Britain and America and I discovered that there wasn't much at all. And then I discovered that the data that did exist contradicted nearly all of the theories including Marx and Ricardo. And then I started to look at other countries and I saw a pattern beginning to emerge, which is that capital, and the money that it produces, accumulates faster than growth in capital societies. And this pattern, which we last saw in the 19th century, has become even more predominant since the 1980s when controls on capital were lifted in many rich countries."