Natural law theory is a philosophical and legal belief that all humans are governed by basic innate laws, or laws of nature, which are separate and distinct from laws which are legislated. Legislated laws are sometimes referred to as “positive laws” in the framework of natural law theory, to make a clear distinction between natural and social laws. This theory has heavily influenced the laws and governments of many nations, including England and the United States, and it is also reflected in publications like the Universal Declaration of Human Rights.
The origins of natural law theory lie in Ancient Greece. Many Greek philosophers discussed and codified the concept of natural law, and it played an important role in Greek government. Later philosophers such as St. Thomas Aquinas, Thomas Hobbes, and John Locke built on the work of the Greeks in natural law theory treatises of their own. Many of these philosophers used natural law as a framework for criticizing and reforming positive laws, arguing that positive laws which are unjust under the principles of natural law are legally wanting.
You are already familiar with natural theory, although you may not be aware of it. Universal ideas about fairness which cross cultural divides are an excellent example of natural law. Many children, for example, appeal to a sense of fairness in disputes, and most people around the world agree that murder is a severe infraction of natural law. Many natural law theorists root their theory in the idea that all humans are essentially reasonable, and that their motives are driven by a sense of self preservation.
Many “natural rights” which are codified in legal language are also a part of natural law, although some theorists argue that humans may give up certain rights to live in society, for the better human good. However, basic tenets of equality and a desire to do good still remain. Some people also integrate religious beliefs into natural law theory, while others refer more generally to basic moral laws which may or may not be guided by religious faith.
There are numerous branches of this theory, some of which are quite complex. Many of these branches use natural law as a framework for discussing positive law, and some of these branches are actually built into legal systems. In England, for example, members of parliament may appeal to natural law theory in settling disputes, in the form of the Fundamental Laws of England, a series of basic rights set out by William Blackstone in the 1760s.