mean the quantity of labour for which the thing will exchange, but the quantity required for producing it. This, they mean to affirm, determines its value; causes it to be of the value it is, and of no other. But when Adam Smith and Malthus say that labour is a measure of value, they do not mean the labour by which the thing was or can be made, but the quantity of labour which it will exchange for, or purchase; in other words the value of the thing, estimated in labour. And they do not mean that this regulates the general exchange value of the thing, or has any effect in determining what that value shall be, but only ascertains what it is, and whether and how much it varies from time to time and from place to place. To confound these two ideas, would be much the same thing as to overlook the distinction between the thermometer and the fire.
