Over at normblog, Norman Geras and Mark Harvey question Marx’s theories regarding where value arises within capitalist systems:
Labour-power, cornerstone of his theory of value and exploitation, is not amenable to being described or theorized by reference to some standard, universalizable labouring-situation, which might then be captured in a mathematically expressible schema. For there is no transcendent situation of the labourer in capitalist economies, and one of the reasons there is not is that the circumstances of those who work and the pressure upon them to exchange their labour-power are never purely economic facts, as they might be within a closed economic system. The worker’s position within capitalist formations is subject to moral, legal and cultural constraints, which themselves contribute to defining the price for which the commodity labour-power is sold and the conditions under which and manner in which it may be used by its purchasers. Moreover, the production of labour-power cannot be treated as comparable to the production of other commodities.
Full text here.