First, it's a post-Marxist book. Marx was writing nearly 200 years ago, after all. The available base of politico-economic knowledge was much smaller then, and the world had had much less experience with the effects of industrialization. Marx was a great genius, but his insights were limited by the knowledge base of his time. Nevertheless, his thought formed the major basis for socioeconomic theory ever since. Even the language of "capitalism" had its origins in Marx, and all subsequent writers, no matter where they positioned themselves on the theoretical spectrum, had to define their positions in terms of Marx.
The dominant trend in economic theory, at least since World War II, was essentially to reject the "messy," qualitative sociological aspects of the field in favor of a hypertrophied set of mathematical models that fed on the more readily quantified aspects of our economic existence. This distortion caused us to reduce all values to monetary measurement, and produced models that became further and further separated from the experienced reality of daily life. It was this systematic, self-inflicted blindness to the fullness of the human experience that permitted economists like Milton Friedman to get away with postulating greed as a virtue.
So along comes Piketty. Yes, he's a post-Marxist. He doesn't have "Marxist credentials." What he does, however, is to broaden the scope of his concerns to something closer to the fullness of human experience, thereby shifting the public consciousness back into an awareness of the things that really matter in the politico-economic sphere.
He does this by attending to a wider set of variables than the economists of the last 70 years or so have acknowledged as important in their models--things like the growing inequality between the classes. He is able to do this in ways that traditional economists find either persuasive or highly threatening (depending on their sociopolitical allegiances) because he has found ways to assign numerical values to these previously unmeasured factors--a range of variables that the Marxists had previously only been able to grapple with in qualitative terms, and the post-war mathematical economists had ignored in their models as unquantifiable.
Thus, in some meta-theoretical sense, he provides a synthesis of the qualitative and humanistic Marxist doctrines and the numerically-driven modeling approaches of the mathematical economists, thereby producing a new and more satisfying level of politico-economic theory.
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