The idea of pure legal theory was put forward by the impressive Austrian jurist and philosopher Hans Kelsen (1881-1973) (see bibliographical note). Kelsen began his long career as a legal theorist in the early 20th century. According to Kelsen, the traditional legal philosophies of the time were hopelessly contaminated by political ideology and moralization on the one hand or by attempts to reduce the right to the natural or social sciences on the other. He found both of these reductionist efforts seriously flawed. Instead, Kelsen proposed a “pure” legal theory that would avoid reductionism. The jurisprudence advocated by Kelsen “is characterized as a `pure` legal theory, because it aims at knowledge oriented solely towards law” and this purity serves as a “basic methodological principle” (PT1, 7). Analytical or “clarifying” jurisprudence means adopting a neutral point of view and using descriptive language when dealing with different aspects of legal systems. It was a philosophical development that rejected the fusion of what law is and what it should be by natural law.  David Hume argued in A Treatise of Human Nature that people inevitably move from describing what the world is to asserting that we should therefore follow a certain course of action. But out of pure logic, we can`t conclude that we should do something just because something is. The analysis and clarification of how the world is must therefore be treated as a strictly normative and evaluative question about what to do. A useful way to get to the heart of these developments is to understand as central the debate on the nature of law, which deals with the relationship between social facts, moral facts and legal content.
Our question is “What determines legal content?”, “legal content” is simply understood as the content of legal norms. Be that as it may, even if Kelsen was wrong about the details of the unity of legal systems, his most important point of view remains true and very important. It is true that the law is essentially systematic, and it is also true that the idea of legal validity and the systematic nature of the law are very closely linked. Standards are legally valid in a particular system, they must be part of a system of standards in force at a given place and time. Whether these two assumptions are actually true is controversial. Joseph Raz argued that they were both inaccurate at best. Two norms may derive their validity from the same fundamental norm, but do not belong to the same system as, for example, an ordered secession, in which a new legal system is created by the legal authorization of another. Nor is it necessarily true that all legally valid norms of a particular system derive their validity from the same fundamental norm (Raz 1979, 127-129). The term “jurisprudence” has been used in different senses at different times. Sometimes it has been used as a synonym for the term “law”, sometimes as a philosophy of law, and sometimes as jurisprudence.
The current trend seems to prefer the term “legal theory” to the term “jurisprudence”. Bentham was also known to call natural law “nonsense on stilts.” It is difficult to know where the positivist tradition begins. Hobbes` theory of law shares certain characteristics with the theories of Jeremy Bentham and John Austin – both clearly in the positivist tradition. Jeremy Bentham developed a very sophisticated version of legal positivism, but for various reasons the most influential and well-known view was that of Bentham`s pupil, John Austin, author of The Province of Jurisprudence Determined (1861). For the purposes of this comprehensive overview, we could use the Latin phrase lex injusta est non lex as a starting point. Natural law theory could be understood as confirming something like this: contemporary debates about the nature of law focus on a revised set of positions. Legal positivism is represented by analytic legal positivists such as H.L.A. Hart, Joseph Raz and Jules Coleman.
The tradition of natural law is defended by John Finnis. And a new position, interpretivism, is represented by the work of the late Ronald Dworkin. In recent years, debates about the nature of the law have become increasingly sophisticated. An important debate is legal positivism. A school is sometimes called “exclusive legal positivism” and is associated with the idea that the legal validity of a norm can never depend on its moral rectitude. A second school is called “inclusive legal positivism”, whose main promoter is Vil Valukhov, and is associated with the idea that moral considerations can determine, but not necessarily, the legal validity of a norm.