Hart was a British philosopher who was a professor of jurisprudence. Hart modifies the theory of Austin and Kelsen. He defined the legal system as such in his book “The Concept of Law”.

In his book The Concept of Law, Hart has analyzed the relation between law, coercion, and morality, and has also attempted to clarify the question of whether all laws may be properly conceptualized as coercive orders or as moral commands.

A legal system is a system of rules which are social in nature because firstly they regulate the conduct of a member of society and secondly, they drive from human social practices”.

The Second reason in his definition gives an idea that in a legal system not only legal rules but also non-legal rules also exist, e.g. morality, customary practices, ethics, values, etc. “Where there is a law, their human conduct is made in some non-optional or obligatory.” Thus the idea of obligation is at the core of the rule.

Austin rejected the content of morality but for Heart, rules are derived from the social practices. Prof. Hart maintains the difference between the source and the relationship between law and morality. He directly accepted the relationship between law and morality, which Kelson tries to keep the Purity of law: Prof. Hart accepted the content of some other elements in law.

Kinds of rules:-

According to Heart, Rules of Obligation are distinguishable from other rules in that they are supported by great social pressure because they are felt to be necessary to maintain society. Our conscience also imposes an obligation.

Having said this he talked about two kinds of rules;

1. Primary rules and 

2. Secondary rules

Primary rules:-

Primary Rules are those rules which impose ‘duty’ on a member of society like criminal laws, tort, etc.

Primary rules are one which tells people to do things, or not to do things. Primary rules are ‘duty imposing’ rules. They impose certain specific duties on the citizens of the state to act in a certain manner, or they may be subject to certain legal sanctions. Hart characterizes primary rules as “basic” rules. They tell the citizen what one can and cannot do under the law. They lay down duties. These rules are to do with physical matters.

Secondary rules:-

Secondary rules are ones which let people, by doing certain things, introduce new rules of the first kind, or alter them. They give people (private individuals or public bodies) the power to introduce or vary the first kind of rule. Secondary rules are not duty-imposing rules. They are what Hart calls power-conferring rules.

Secondary rules are those rules which confirm ‘powers’ like Contract, Marriage, Will, Delegated Legislation – the power to make law.

In the Indian Constitution, Schedule VII gives a list namely State, Centre, and Concurrent List, which conferred power to respective organs to make laws. There is a link between these primary and secondary rules. There is a specific relationship between these rules which rather systematically comprises a legal system and legal order.

Secondary rules have been divided into three more types, these are as follows;

1. Rule of Adjudication

2. Rule of Change

3. Rule of Recognition

Rule of Adjudication:-

It mainly represents those rules, which confer a direct power to adjudicate the matter in dispute, e.g. Article 32, which empowers Supreme Court to issue prerogative writ: Article 131, 132, 134, 133 that empower Supreme Court the original and Appellate jurisdiction. Article 323A and 323 B empower tribunals to adjudicate matters in dispute. All those articles in the Constitution are power conferring. They enable a court to decide a particular dispute.

Rule of change:-

Law-making power is to be accompanied by modification when a competent legislative body derived its power to make law and should have the power to change the law. This power is necessary to affect any kind of notification, e.g. Article 368 gives power to Parliament to amend the Constitution and procedure thereof. Thus it gives the power to amend the Constitution. This power includes the power to repeal, and remove difficulties. It is equally applicable to delegated legislation.

Rules of recognition:-

The rule of recognition is the criterion of existence and validity of the rule of the legal system. Hart believes that the rule of recognition is the most important. The rule of recognition tells us how to identify a law. In the modern system with multiple sources of law such as a written constitution, legislative enactments, and judicial precedents, the rules of recognition can be quite complex and require a hierarchy where some types of rules overrule others. Hart holds this out as the remedy for uncertainty.

 Criticism by professor Dwarkin

Prof. Hart called a legal system a system of Rules. Whether a legal system is a system of rules only? Prof. Ronald Dwarkin criticized Prof. Heart on this point. Dwarkin pointed out that the legal system does not comprise only rules but it consists of principles also. So to call the legal system of rules is not proper. Sometimes those principles are more important than those rules, e.g. Principle of Natural Justice, which is elaborated in Maneka Gandhi vs. Union of India. The judiciary positively incorporates the Principle of Natural Justice. So what Dwarkin says is also an important one. If rules and principles come into conflict then principle gets primacy with overriding effect overrules.

Justice Coke in Bohman’s Case (1610), contended that “if it is found that the law made by Parliament is contrary to certain moral principle then such law could be null and void”.