Piketty outlines the case for the inequality r>g (rate of return on capital > growth of overall economy) with extensive documentation. From this simple rule he describes the forces which lead to the rise of a rentier class that can live very well off inherited wealth which isn't necessarily socially useful. Ultimately I disagree with his solutions to this problem but it seems fairly hard to challenge the idea that wealth above a certain level tends to reproduce itself. Reinvestment of a portion of the returns on capital leads to a natural process of accumulation which Piketty argues will lead to a dangerous level of wealth concentration in the hands of a very few. Throughout the book he's very open about the strengths and weaknesses of his sources and openly declares his personal opinion about wealth accumulation. He did a tremendous amount of research and analysis and that's the main reason for my high rating: he backed up his opinions with historical and modern data. The policies he proposes are debateable but he establishes the premise (r>g) in extensive detail and across a broad range of countries and time periods. Few would accuse him of using too little data and historical sources.