Judges, legislators and legal academics are responsible for identifying what the law is by using logic and reason to interpret statutes or case law.
Legal formalists argue that if judges innovate new law, they violate the separation of powers. Judges interpreting the Constitution or statutes in the United States are guided by principles of legislative interpretation. The U.S. Supreme Court stated in Connecticut National Bank v. Germain, 112 S. Ct. 1146, 1149 (1992), "Courts must presume that a legislature says in a statute what it means and means in a statute what it says there." The legislature can issue guidance to clarify legislation, either contemporaneously or after an act has been passed. This guidance is generally not binding. It is assumed that the legislature uses ordinary English words in their ordinary sense. Legal terms, concepts and statutory provisions that existed prior to new legislation are assumed not to have changed, unless the legislation clearly specifies otherwise.
If legislation is ambiguous, or the result of a simple reading would be absurd, judges can turn to canons of interpretation, developed by the common law. Examples include the proposition that when a list of specific items is followed by a general rule, the otherwise wide meaning of the general rule must be restricted to the specific items (ejusdem generis). Ambiguities can be ironed out by looking at the context of the statute in question (noscitur a sociis) or by using other statutes (in pari materia). The courts will defer to an agency's reasonable interpretation of ambiguous statutes. When two statutes conflict, the one enacted last prevails.
U.S. judges have held that national statutes should be read consistently with international law (Murray v. The Charming Betsy, 6 U.S. (2 Cranch) 64, 1804) and must be presumed not to violate societal values.
The common law in the United States, England and other common law jurisdictions including Australia, India, Ireland, Malaysia, New Zealand, Pakistan, South Africa, Sri Lanka, and Zimbabwe, are based on the principle of precedent (stare decisis). The way that a case was decided in the past or by a higher judge raises a presumption that any similar case must be decided the same way. Absent special considerations, judges are bound by prior decisions, unless they can be distinguished.
Legal realists such as Roscoe Pound (1870–1964), Wesley Hohfeld (1879--1918) and Justice Oliver Wendell Holmes (1841–1935) considered that legislative interpretation and precedent seldom determined legal disputes. Judges interpret the law to achieve what they believe is mandated by social justice (legal instrumentalism). For example, judges developed civil rights in the absence of legislation such as ending segregation in the American education system (Brown v. Board of Education of Topeka, 347 U.S. 483, 1954). They did so on the basis of their understanding of what the law should be, rather than what the intention of the legislature was when the law was enacted.
Thomas Aquinas (c.1225--1274) was a proponent of natural law which argues that law represents a basic universal truth that can be discovered through logic and reason. This was rejected by legal philosophers such as John Austin (1790--1859) who suggested that law was whatever a person in power enacted that could be enforced. Herbert Hart (1907--1992) developed the theory of legal positivism. Hart rejected the idea that law is necessarily moral or rational, or that there is any universally true law, but posited that law simply represents the outcome of the legislative and judicial process.
The law as integrity model of Ronald Dworkin (1931--) argues that when judges make value judgments, such as in "hard" civil rights cases, they are only doing so in an honest attempt to understand what the law is and to apply it to the facts of the case. He suggested that despite gaps in the law, or inconsistencies, judges would arrive at one conclusion, even in "hard" cases.
Hohfeld is also well known for Hohfeldian analysis of rights and powers as being corollaries of duties and liabilities respectively. He distinguished these from the "Jural Opposites" of no rights and disabilities. A practical application of this is that the right to education implies a duty on the part of the state to provide education. The power of the state to enforce school attendance implies a corresponding liability on the part of recalcitrant parents. The opposite of a right to education would be no right. The opposite of the power to enforce attendance would be to disable parents from having their children attend school.