We have already noticed that the conception of the “natural order” underwent considerable modification during the period which intervened between the writings of the Physiocrats and the appearance of the _Wealth of Nations_. The Physiocrats regarded the “order” as one that was to be realised, and the science of political economy as essentially normative. For Smith it was a self-realising order. This spontaneity of the economic world is analogous to the vitality of the human body, and is capable of triumphing over the artificial barriers which Governments may erect against its progress. Practical political economy is based upon a knowledge of the economic constitution of society, and its sole aim is to give advice to statesmen. According to Say, this definition concedes too much to practice. Political economy, as he thinks, is just the science of this “spontaneous economic constitution,” or, as he puts it in 1814, it is a study of the laws which govern wealth.[263] It is, as the title of his book suggests, simply an exposition of the production, distribution, and consumption of wealth. It must be distinguished from politics, with which it has been too frequently confused, and also from statistics, which is a simple description of particular facts and not a science of co-ordinate principles at all. Political economy in Say’s hands became a purely theoretical and descriptive science. The _rôle_ of the economist, like that of the savant, is not to give advice, but simply to observe, to analyse, and to describe. “He must be content to remain an impartial spectator,” he writes to Malthus in 1820. “What we owe to the public is to tell them how and why such and such a fact is the consequence of another. Whether the conclusion be welcomed or rejected, it is enough that the economist should have demonstrated its cause; but he must give no advice.”[264]