Law is the system of rules and regulations established in a society to regulate its behaviour. Its fundamental source is the constitution or statutes of a state or nation, but it also includes a body of custom and policies recognised and enforced by judicial decision.
As a practical matter, laws may be either positive or negative; they are not necessarily abstract or theoretical, as some philosophers might claim (for example, an ideal of justice). Most people will encounter both, in daily life: positive laws regulate such things as traffic laws and building codes, whilst negative laws prohibit blasphemy or adultery.
Most countries use the civil law system, in which the sources recognised as authoritative are primarily legislation and custom (as codified by statutes passed by government). In this system courts decide cases according to precedent. The principle of stare decisis is a pillar of the civil law, and enables the court system to maintain consistency and predictability.
Other legal systems are less strictly procedural, and allow more scope for interpreting case law in a wide variety of ways. The common law, for instance, relies more on judicial interpretation of earlier decisions and of the natural consequences of certain actions. This makes it more difficult to predict how a particular judge might decide in a given case.
In the modern world, laws cover a range of areas from social security to space exploration. International law concerns the relations of countries with each other, and this includes treaties regulating human activities in outer space. Domestic law covers issues such as marriage and divorce, and property rights in the event of separation. Tax law and banking regulation establish minimum standards for capital and rules about investment.
There is also a broad range of specialisms in law: employment law covers the relationships between employers and employees, and immigration law and nationality law deal with the rights of people who are not citizens of the country they live in. In addition, family law covers the rights of married and unmarried couples, and private and public sector utility companies must adhere to a range of regulations in the areas of water, energy and transport.
The philosophy of law encompasses a vast range of ideas about how the nature and purposes of laws should be understood. Aristotle developed his theory of law based on the premise that laws are general rules of action, enacted by legislation and revisable through direct popular vote or other plebiscitary means. More recently, Max Weber and others have reshaped thinking about the extension of the power of the state beyond its traditional military, police and bureaucratic functions into areas such as education, health and welfare.
Law also refers to the profession of lawyers, and the study of systems of law and their operation. The practice of law requires a great deal of intellectual effort, and there are few careers more challenging or rewarding than that of a lawyer. Lawyers need to be both articulate and able to work well in teams, and they should be willing to make sacrifices in order to do their jobs effectively.