For all the power the Government has being only for the good of society, as it ought not to be arbitrary and at pleasure, so it ought to be exercised by established and promulgated laws; that both the people may know their duty, and be safe and secure within the limits of the law.
—John Locke. Two Treatises of Government, 1689.
The rule of law requires that laws be known before they take effect, meaning they must be written, precise, accessible, well-publicized, and never retrospective. Laws should be bills of particulars, never abstract concepts which the judiciary or bureaucrats are left to interpret according to the ‘changing mores of society.’ Ayn Rand explained:
‘Fluid law’ is a euphemism for ‘arbitrary power’—that ‘fluidity’ is the chief characteristic of the law under any dictatorship—and that the sort of ‘dynamic society’ whose laws are so fluid that they flood and drown the country may be seen in Nazi Germany or Soviet Russia. (Capitalism: The Unknown Ideal, 1966.)
Fluid laws cause citizens to live in fear, both for their person and their property, never knowing when a judge will fancy that the mores of society have changed against them and that an action, once legal, will, upon prosecution, be deemed criminal. Fluid law comes about whenever the government runs businesses, bails out banks or companies or otherwise oversteps its proper bounds. All these adventures make government actions more unpredictable, which in turn makes it more difficult for individuals to lay plans for opening a business, buying a home, or otherwise pursuing happiness. Cesare Beccaria noted how uncertain laws are the harbingers of tyranny:
I do not know of any exception to this general axiom, that every member of society should know when he is criminal and when innocent. If censors, and, in general, arbitrary magistrates, be necessary in any government, it proceeds from some fault in the constitution. The uncertainty of crimes hath sacrificed more victims to secret tyranny than have ever suffered by public and solemn cruelty. (Of Crimes and Punishments, 1764.)
Certainty of law is equally important in civil law; knowing how contracts will be interpreted and how to avoid government penalties allows people to invest with confidence and foresight. As Friedrich Hayek observed:
There is probably no single factor which has contributed more to the prosperity of the West than the relative certainty of the law which has prevailed here. This is not altered by the fact that complete certainty of the law is an ideal which we must try to approach but which we can never perfectly attain. It has become the fashion to belittle the extent to which such certainty has in fact been achieved, and there are understandable reasons why lawyers, concerned mainly with litigation, are apt to do so. They have normally to deal with cases in which the outcome is uncertain. But the degree of the certainty of the law must be judged by the disputes which do not lead to litigation because the outcome is practically certain as soon as the legal position is examined. It is the cases that never come before the courts, not those that do, that are the measure of the certainty of the law. (The Constitution of Liberty, 1960.)