Capital in the 21st Century by Thomas Piketty

"There is, however, a central difficulty with Piketty’s argument. It rests on a mistaken definition of capital. Capital is a process not a thing.

It is a process of circulation in which money is used to make more money often, but not exclusively through the exploitation of labor power.

Piketty defines capital as the stock of all assets held by private individuals, corporations and governments that can be traded in the market no matter whether these assets are being used or not"

David Harvey
See Also :
http://democracyandclassstruggle.blogspot.co.uk/2014/05/afterthoughts-on-pikettys-capital-by.html