This is what prevents the economist from making any exact predictions about the effect of the _vis mediatrix reipublicæ_. He may, like Adam Smith, find it doing good work in the undermining of feudalism,[237] and he may point out that at any rate it would make a better guide to the world than military glory, which means unhappiness to one-half the world, and a very mingled happiness to the other half. But he cannot predict its effect on men whose characters are unknown to him. He cannot even tell whether a man is wealthy or not, till he knows what his wants are, for wealth exists to satisfy wants, wants change with human progress, the notion of wealth expands with civilization, and the luxuries of one age and one man are the necessaries of another. It is impossible to treat this relative question as if its conditions were absolute, and to deal with men as we would with figures on a slate. Two and two do not always make four in such a case, but sometimes five, and frequently only three. A new vista of comfort spread before different men may stimulate one, spoil another, and leave a third unmoved. It is not surprising then that the question, “By what various modes is population kept to the level of the food in the states of modern Europe?” is not a simple one. On some grounds it would seem comparatively easy to get the answer. There are figures to be had, and in many cases a census; there is a general similarity of circumstances which produces a general similarity of habits, and, therewith, of the movements of population. But there is no invariable order of mortality and generation. The rates of births and deaths are not the same for all nations; they depend on the conduct of human beings, and may differ not only in different countries, but in different parts of the same country. In the same way, we have no single statistical criterion of the healthy state of a population, just as it might be said we have no single criterion of the commercial prosperity of a country, still less of its happiness. The two former stand to the last as the parts to the whole. A healthy population and a prosperous trade are parts of the happiness of a nation, though they do not constitute the whole of it. To ascertain whether a nation is happy or not, we have to take into account these two parts of happiness along with many others. The parts in their turn consist of many parts. We measure the state of trade not only by imports and exports, railway, banking and Clearing House returns, and the gains of the public revenue, but by subscriptions to churches, charities, and schools, by savings banks and benefit societies, sales of books, pictures, and luxuries of all kinds, by the state of workmen’s wages, by the poor-law returns, by the number of marriages, emigrants, and recruits for the army; and we could make little use of most of these figures without the census returns and the reports of the Registrar General.