Surplus labour creates surplus value, and on Marx’s analysis, surplus value is the source of all profit. It is this that makes the difference between the money advanced and the money received. The process of ‘extracting’ surplus value is called ‘exploitation’. Finally we have arrived at the point of Marx’s great discovery. Under capitalism all profit is ultimately the result of the exploitation of the workers. For, by this account, there is simply nowhere else for profit to come from.
Now you may think, either this represents the workers as very stupid, or there must be something wrong with Marx’s analysis. For if workers posses this incredibly valuable thing— labour power—why don’t they keep it to themselves, or, at least sell it for a decent price?
The reason why they don’t keep it to themselves and thus harness its full earning potential, says Marx, is that they can’t. According to Marx one of the conditions of capitalism’s existence is that there must be a class of workers who are free in an ironic ‘double sense’. First, they must be free from feudal ties, which would otherwise prevent them from entering any sort of market transaction. Second, they must be ‘free’ from independent access to the means of production. In other words they must both be able to work for capitalists and need to. They acquiesce in their own exploitation only because they have no alternative. They cannot work for themselves as they have nothing to work on or with, no land or other resources. Thus they must hire out their labour power to the highest bidder.