[39] It might appear at first that the market price of labour expressed such an exchange: but this is a fallacy, for the market price is the momentary price of the kind of labour required, but the just price is its equivalent of the productive labour of mankind. This difference will be analysed in its place. It must be noted also that I speak here only of the exchangeable value of labour, not of that of commodities. The exchangeable value of a commodity is that of the labour required to produce it, multiplied into the force of the demand for it. If the value of the labour = _x_ and the force of demand = _y_, the exchangeable value of the commodity is _xy_, in which if either _x_ = 0, or _y_ = 0, _xy_ = 0. If we promise to give him less labour than he has given us, we under-pay him. If we promise to give him more labour than he has given us, we over-pay him. In practice, according to the laws of demand and supply, when two men are ready to do the work, and only one man wants to have it done, the two men under-bid each other for it; and the one who gets it to do, is under-paid. But when two men want the work done, and there is only one man ready to do it, the two men who want it done over-bid each other, and the workman is over-paid. I will examine these two points of injustice in succession, but first I wish the reader to clearly understand the central principle lying between the two, of right or just payment.