Public law is sub-divided into several branches, including constitutional, administrative and statutory law, which resolve conflicts between individuals and government. Public law was originally defined by the Romans as res publica — that is "the public thing," or in the public interest and common good, and based on the differentiation between the State and the government. Under the Romans, res publica was an alternative to the traditional ruling systems, where the power was vested entirely in the hands of a single person. Rome in the 1st century B.C.E. had no written constitution, but the republic used unwritten guidelines and principles passed down mainly through precedent.
Res publica, public law, as the thing of people, was first defined by the Roman senator Cicero. Res publica usually refers to a thing that is not considered to be privately owned (res privata) but which is held in common by many people. A park or garden in the city of Rome could either be "private property," or managed by the state, in which case it would be res publica. Initially, Rome's public law was closely related to religion, but over time this connection weakened.
After the Roman Empire, the concept of public law was adopted by monarchies and republics. In monarchies, public law represented the power of the monarch, while in republics public law was a responsibility of the people. In France, the concept of public law was represented by the idea of the State (l'Etat). The French State comprises the community of permanent interests of the nation, rather than a tool for domination. Various concepts such as the public interest derive from the French idea of public law.
In Germany, public law developed around the concept of the Prince State (Furstenstaat). This personified idea of the State had a great influence on the development of public law in general. The idea of the Prince State lost its popularity in the 19th century and in the 20th century it remained only in theory, for example in the works of Hans Kelsen.
Unlike continental Europe, England did not embrace the idea of res publica. Instead, the English and Scottish legal systems are based on common law, which evolved over time based on precedent, with lawyers looking to previous court rulings upon which to base their decisions. According to some researchers, the rule of law created by courts tends to protect private interests and is therefore is the opposite of public law. The United Kingdom has never had a written constitution. Its legal system is based on statutes such as:
Magna Carta — the liberties charter from the 13th century
the Petition of Rights
the Bill of Rights
the Act of Settlement (1700)
the Parliament Acts (1911 and 1949)
With the founding fathers of the United States being familiar with the fundamentals of English law, the values of the U.S. legal system are those of common law. The United States did not embrace the idea of public law, in the sense of res publica, but it did adopt a number of public law acts. Only the state of Louisiana makes an exception, following the French tradition of public law, especially the French civil code, known as the Napoleonic Code.
U.S. public law acts include:
the Constitution and the Preamble
the Bill of Rights
the Civil War Amendments
Other English speaking states, such as Australia and the English-speaking provinces of Canada, also tended to adopt the common law model.
Studies into the public law in continental Europe and the United States have examined various factors determining the difference in the legal systems. Some see the main reason in the fact that France, for example, adopted res publica from Rome, whereas the United States did not.