What is sold by him who has to labor in order to live, and who has not in his possession the means of labor, to the possessor of those means is simply labor in the potential state, that is the muscular or intellectual faculties that must be exerted in the production of useful things. In fact, on the one hand, before these faculties are brought into active exercise, labor does not exist and cannot be sold. Now, the contract is made between the buyer and the seller before any action takes place and has for its effective cause, so far as the seller is concerned, the fact that the seller is so situated that he can not by himself bring his capacity for labor into productive use. On the other hand, as soon as the action (labor) begins, as soon as labor manifests itself, it cannot be the property of the laborer, for it consists in nothing but the incorporation of a thing which the laborer has just alienated by sale--capacity to perform labor--with other things which are not his--the means of production. To sum up, when the labor does not exist, the laborer can not sell that which he does not possess and which he has not the means of realizing; when the labor does exist, it can not be sold by the laborer to whom it does not belong. The only thing which the laborer can sell is his labor-power, a power distinct from its function, labor, just as the power of marching is distinct from a parade, just as any machine is distinct from its operations. What is paid under the form of wages by the possessor of the means of labor, the purchaser of the labor-power to the possessor of that power, cannot, therefore, be, and is not, the price of the labor furnished, but is the price of the power made use of, a price that supply and demand cause to oscillate about and especially below its value determined, like the value of any other commodity, by the labor-time socially necessary for its production, or in other words, in this case by the sum which will normally enable the laborer to maintain and perpetuate his labor-power under the conditions necessary for the given kind and stage of production. But, even when the laborer gets a value equal to the value of his power, he furnishes a value greater than that which he receives. The duration of labor required for a given wage, regularly exceeds the time necessarily occupied by the laborer in adding to the value of the means of production consumed, a value equal to that wage; and the labor thus furnished over and above that which represents the equivalent of what the laborer gets, constitutes _surplus-labor_. SURPLUS-LABOR THEN IS UNPAID LABOR.