Natural Law, Legal Positivism, and Legal Realism want to know the 5 W's of these terms who, what, where, when, and why. 

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Natural law, legal realism, and legal positivism are three ways of thinking about the origins of law and its sources (or lack thereof) of legitimacy.  These theories are not necessarily reducible to the “Five W’s.”  In particular, it is not really possible to say that any of them is attributable to any one person.

Natural law is the idea that laws (or at least many laws) exist naturally.  They are not dependent on the actions of any particular government.  Because of this, these laws are universal, applying to all human beings.  For example, a natural law theorist might say that all human beings have the right to life, liberty, and property regardless of whether the government under which they live recognizes these rights.  This idea has its roots in classical times, but it is perhaps most commonly connected with Enlightenment thinkers such as John Locke.  It is most commonly connected with Europe beginning in the mid-1600s.  As to the “why” of natural law, we can say that natural law that monarchs claimed that they had a divine right to rule.  During the Enlightenment, thinkers were looking for ideas about law that did not rely on the dubious idea that God had given power to a certain family.  This helped lead them to the idea of natural law.

Legal positivism and legal realism are reactions against the idea of natural law.  They both hold that law is a social construct.  In other words, they both hold that laws come about because a given society (or the people in power in a given society) wants those laws.  In other words, laws reflect the ideas and prejudices of a given society.  They do not reflect some universal truth.  Legal positivists hold that law exists as a set of formal rules set out by governments.  By contrast, legal realists hold that law is more malleable and depends on changing ideas among the people.  Both of these ideas are most closely connected with the early 20th century.  The main figure in legal positivism was H. L. A. Hart while the most prominent legal realist was Oliver Wendell Holmes, Jr.  Hart was a British scholar and Holmes was American who was a judge and served on the Supreme Court of the United States.  Both of these theories arose out of a certain degree of jadedness.  Natural law theory implies that laws come from a universal sense of morality.  Legal realists and positivists were more inclined to be cynical and to say that laws come about because people want them to exist.