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.