The rule of law is the “supremacy of regular power as opposed to arbitrary power”. The phrase can be traced back to the 17th century, and it was popularized in the 19th century by British jurist A. V. Dicey. Even earlier the phrase “Not under man but God and Law” was attributed to the British jurist Henry of Bracton while the concept was familiar to ancient philosophers as well.
Aristotle wrote that “Law should govern.” as well as “The only stable state is the one in which all men are equal before the law”.  Rule of law implies that every citizen is subject to the law. It stands in contrast to the idea that the ruler is above the law, for example by divine right.

In its general sense, the phrase can be traced back to the 16th century, and it was popularized in the 19th century by British jurist A. V. Dicey. The concept was familiar to ancient philosophers such as Aristotle, who wrote “Law should govern”.[4] Rule of law implies that every citizen is subject to the law. It stands in contrast to the idea that the ruler is above the law, for example by divine right.

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Bracton saith, quod Rex non debed esse sub homine, sed sub Deo et lege (That the King ought not to be under any man but under God and the law.).”

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The lead definition given by Black's is this: “A substantive legal principle”, and the second definition is the “supremacy of regular as opposed to arbitrary power”.

Schematically man can say that the rule of law is a system in which the following four universal principles are upheld:
o         The government and its officials and agents are accountable under the law.
o          The laws are clear, publicized, stable and fair, and protect fundamental rights, including the security of persons and property.
o          The process by which the laws are enacted, administered, and enforced is accessible, efficient, and fair.
o          Justice is delivered by competent, ethical and independent representatives and neutrals, who are of sufficient number, have adequate resources, and reflect the makeup of the communities they serve.

As used by the World Justice Project, a non-profit organization committed to advancing the rule of law around the world, the rule of law refers to a rules-based system in which the following four universal principles are upheld:[56]
1. The government and its officials and agents are accountable under the law;
2. The laws are clear, publicized, stable, fair, and protect fundamental rights, including the security of persons and property;
3. The process by which the laws are enacted, administered, and enforced is accessible, fair, and efficient;
4. Access to justice is provided by competent, independent, and ethical adjudicators, attorneys or representatives, and judicial officers who are of sufficient number, have adequate resources, and reflect the makeup of the communities they serve.

Nowadays it is safe to say that  the rule of law ultimately comes down to a principle of governance in which all persons, institutions and entities—including the State itself—are accountable to laws that are publicly promulgated, equally enforced and independently adjudicated—and which are consistent with international human rights norms and standards.

The Secretary-General of the United Nations defines the rule of law as:[47]
a principle of governance in which all persons, institutions and entities, public and private, including the State itself, are accountable to laws that are publicly promulgated, equally enforced and independently adjudicated, and which are consistent with international human rights norms and standards.

In European Union law there are generally acknowledged to be four stages to a proportionality test, namely: there must be a legitimate aim for a measure or restriction
o    the measure or restriction must be suitable to achieve the aim (potentially with a requirement of evidence to show it will have that effect)
o    the measure or restriction must be necessary to achieve the aim, that there cannot be any less onerous way of doing it
o    the measure or restriction must be reasonable, considering the competing interests of different groups at hand.

In European Union law there generally acknowledged to be four stages to a proportionality test, namely,[3]

• there must be a legitimate aim for a measure
• the measure must be suitable to achieve the aim (potentially with a requirement of evidence to show it will have that effect)
• the measure must be necessary to achieve the aim, that there cannot be any less onerous way of doing it
• the measure must be reasonable, considering the competing interests of different groups at hand

Two years earlier in Hamdi v. Rumsfeld case the Court had recognized the power of the government to detain enemy combatants, including U.S. citizens, but ruled that detainees who are U.S. citizens must have the rights of due process, and the ability to challenge their enemy combatant status before an impartial authority.

It took us decades of hard negotiations to move forward from the European Community of Coal and Steel (1952) to the European Economic Community (1957) and from it to the European Union (1992). The latter constitutes today an economic and political union of 27 member states encompassing a population of more than 500 million habitants and ranking No 1 in the list of countries by Gross Domestic Product (GDP) in total (Per capita ranks No 15).

The EU traces its origins from the European Coal and Steel Community (ECSC) and the European Economic Community (EEC), formed by the Inner Six countries in 1951 and 1958 respectively.

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With a combined population of over 500 million inhabitants,[24] or 7.3% of the world population,[25] the EU, in 2012, generated a nominal gross domestic product (GDP) of 16.584 trillion US dollars, representing approximately 20% of the global GDP when measured in terms of purchasing power parity, and represents the largest nominal GDP and GDP PPP in the world.[26]

Fundamental rights established by the European Convention for the Protection of Human Rights and Fundamental Freedoms (1950) and other international legal instruments ratified by the Members States, as well as those deriving from the constitutional traditions common to them, form part of the general principles of law that are enforceable by the Court of Justice of the European Union.

Moreover the Treaty of Lisbon (2007) brought substantial amendments to the field of human rights environment by giving them a legally binding status, making them part of primary law and providing EU citizens with an additional layer of protection:

The revised Charter of Fundamental Rights originally proclaimed in Nice (2000) is the most updated and comprehensive legal instrument of human rights in the world. It contains fifty “rights, freedoms and principles”, stretching from civil and political rights on the one hand to economic, social and cultural rights on the other. Being part of the primary law, the Charter prevails over national law of Member States, when they are acting within the scope of EU law (Article 51). After the adoption of the Treaty of Lisbon, it became directly enforceable by the EU and national courts (Article 51(1)).

It is important to keep in mind that fundamental rights established by the European Convention for the Protection of Human Rights and Fundamental Freedoms, and other international legal instruments ratified by the Members States, as well as those deriving from the constitutional traditions common to them, form part of the general principles of law that are enforceable by the Court of Justice of the European Union.

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The Treaty of Lisbon brought substantial amendments to the field of human rights environment. The Charter of fundamental rights of the European Union (hereinafter referred as the ‘Charter’) is now part of primary law and has been given a legally binding status.
The prime objective of this Charter is to bring together your European citizens’ rights that had been scattered over a range of national and international instruments, making them more accessible, visible while providing EU citizens with an additional layer of protection.
The revised Charter now contains fifty “rights, freedoms and principles” the Charter contains both civil and political rights on the one hand, and economic, social and cultural rights on the other. There are also a number of new generation’s rights – such as the right to a clean environment and a right to good administration. These rights are organised under five major headings – human dignity, fundamental freedoms, equality, solidarity, citizenship and justice. It also contains four horizontal provisions, which are intended to clarify its field of application, scope and interpretation of its provisions.
Applying to the EU institutions and bodies, it means that they must conform to the Charters’ rights and observe its principles.
Being part of the primary law, the Charter prevails over national law of Member States, but only when they are acting within the scope of EU law (Article 51). After the adoption of the Treaty of Lisbon, it became directly enforceable by the EU and national courts (Article 51(1)).]

Especially the Prüm Convention, also known as Schengen III Agreement (2005) has been criticized as the ‘eye of Big Brother over Europe’. The Treaty provides the exchange of data regarding DNA, fingerprints and vehicle registration of concerned persons and cooperation against terrorism. It also contains provisions for the deployment of armed sky marshals on flights between signatory states, joint police patrols, entry of (armed) police forces into the territory of another state for the prevention of immediate danger (hot pursuit), and cooperation in case of mass events or disasters.
The Treaty was initially adopted outside the EU framework but some of its provisions have already been subsumed into the police and judicial cooperation provisions of EU law by Council decisions providing for law enforcement cooperation in criminal matters primarily related to exchange of fingerprint, DNA (both on a hit no-hit basis) and vehicle owner registration (direct access via the EUCARIS system) data. 

The Prüm Convention (sometimes known as Schengen III Agreement[1]) is a treaty which was signed on 27 May 2005 by Austria,Belgium, France, Germany, Luxembourg, the Netherlands and Spain in the town of Prüm in Germany. The convention was joined later by other members of the Schengen Agreement.
Παρακάτω:
The Convention was adopted so as to enable the signatories to exchange data regarding DNA, fingerprints and Vehicle registration of concerned persons and to cooperate againstterrorism. It also contains provisions for the deployment of armed sky marshals on flights between signatory states, joint police patrols, entry of (armed) police forces into the territory of another state for the prevention of immediate danger (hot pursuit), and cooperation in case of mass events or disasters.

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The Convention was adopted outside of the European Union framework (and its mechanism of Enhanced co-operation), but asserts that it is open for accession by any Member state of the European Union and that:
provisions of this Convention shall only apply in so far as they are compatible with European Union law … [EU law] should take precedence in applying the relevant provisions of this Convention
— Convention on the stepping up of cross-border cooperation, particularly in combating terrorism, cross-border crime and illegal migration, Article 47
Additionally the text of the Convention and its annexes were circulated on 7 July 2005 between the delegations to the Council of the European Union.
Some of the Convention provisions, falling under the former third pillar of the EU, were later subsumed into the police and judicial cooperation provisions of European Union law by 2008 Council Decisions,[3][4] commonly referred to as the Prüm Decision. It provides for Law Enforcement Cooperation in criminal matters primarily related to exchange of Fingerprint, DNA (both on a hit no-hit basis) and Vehicle owner registration (direct access via the EUCARIS system) data.