Rechtsstaat is a doctrine in continental European legal thinking, originating in German jurisprudence, that can be translated as "legal state", "state of law", "state of justice", "state of rights", or "state based on justice and integrity".
A Rechtsstaat is a "constitutional state" in which the exercise of governmental power is constrained by the law, and is often tied to the Anglo-American concept of the rule of law, but differs from it in that it also emphasizes what is just (i.e., a concept of moral rightness based on ethics, rationality, law, natural law, religion, or equity). Thus it is the opposite of Obrigkeitsstaat (a state based on the arbitrary use of power).
In a Rechtsstaat, the power of the state is limited in order to protect citizens from the arbitrary exercise of authority. The citizens share legally based civil liberties and can use the courts. A country cannot be a liberal democracy without being a Rechtsstaat.
German writers usually place the theories of German philosopher Immanuel Kant (1724–1804) at the beginning of their accounts of the movement toward the Rechtsstaat. The Rechtsstaat in the meaning of "constitutional state" was introduced in Kant's latest works after US and French constitutions were adopted in the late 18th century. Kant’s approach is based on the supremacy of a country’s written constitution. This supremacy must create guarantees for implementation of his central idea: a permanent peaceful life as a basic condition for the happiness of its people and their prosperity. Kant proposed that constitutionalism and constitutional government ought to be sufficient to guarantee this happiness. He thus formulated the main problem of constitutionalism: "The constitution of a state is eventually based on the morals of its citizens, which, in its turn, is based on the goodness of this constitution."