James K. Galbraith reviews Thomas Piketty's Capital in the Twenty-First Century in Dissent:
Early in the last century, neoclassical economics dumped this social and political analysis for a mechanical one. Capital was reframed as a physical item, which paired with labor to produce output. This notion of capital permitted mathematical expression of the “production function,” so that wages and profits could be linked to the respective “marginal products” of each factor. The new vision thus raised the uses of machinery over the social role of its owners and legitimated profit as the just return to an indispensable contribution.
Symbolic mathematics begets quantification. For instance, if one is going to claim that one economy uses more capital (in relation to labor) than another, there must be some common unit for each factor. For labor it could be an hour of work time. But for capital? Once one leaves behind the “corn model” in which capital (seed) and output (flour) are the same thing, one must somehow make commensurate all the diverse bits of equipment and inventory that make up the actual “capital stock.” But how?
Although Thomas Piketty, a professor at the Paris School of Economics, has written a massive book entitled Capital in the Twenty-First Century, he explicitly (and rather caustically) rejects the Marxist view. He is in some respects a skeptic of modern mainstream economics, but he sees capital (in principle) as an agglomeration of physical objects, in line with the neoclassical theory. And so he must face the question of how to count up capital-as-a-quantity.
His approach is in two parts. First, he conflates physical capital equipment with all forms of money-valued wealth, including land and housing, whether that wealth is in productive use or not. He excludes only what neoclassical economists call “human capital,” presumably because it can’t be bought and sold. Then he estimates the market value of that wealth. His measure of capital is not physical but financial.
This, I fear, is a source of terrible confusion.
More here.