Ricardo's purpose was to frame a calculus, to give a method of reasoning which will enable us to clinch our economic reasoning. We are to be sure that we have followed out the whole cycle of cause and effect. Capitalists, landowners, labourers form parts of a rounded system, implying reciprocal actions and reactions. The imposition of a tax or a tariff implies certain changes in existing relations: that change involves other changes; and to trace out the total effect, we must understand what are the ultimate conditions of equilibrium, or what are the processes by which the system will adjust itself to the new conditions. To describe, again, the play of a number of reciprocal forces, we have to find what mathematicians call an 'independent variable': some one element in the changes on which all other changes will depend. That element, roughly speaking, ultimately comes out to be 'labour.' The simplicity of the system gave an impression both of clearness and certainty, which was transferred from the reasoning to the premises. The facts seemed to be established, because they were necessary to the system. The first step to an estimate of the value of the doctrine would be to draw up a statement of the 'postulates' implied. Among them, we should have such formulæ as the single rate of profits and wages; which imply the 'transferability' of labour and capital, or the flow of either element to the best-paid employment. We should have again the Malthusian doctrine of the multiplication of labour up to a certain standard; and the fact that scarcity means dearness and plenty cheapness. These doctrines at least are taken for granted; and it may perhaps be said that they are approximations which only require qualifications, though sometimes very important qualifications, to hold good of the society actually contemplated.