The Relation of Ethics to Natural Law

Legal Positivism and Ethics

The relation of ethics to natural law is difficult. Legal positivism states that there is no inherent or necessary connection between the validity of a law on the one hand, and ethics or morality on the other. Therefore, in legal positivism, the law is seen as being conceptually separate (though of course not separated in practice) from moral and ethical values, and it simply sees the law as being posited by lawmakers who are humans.
Although a positivist's view of law is that it is ultimately a matter of human custom or convention, this does not entail or presuppose that positivists endorse laws of any particular content, or the view that valid law is always to be obeyed by citizens or applied by judges. On the flip side, legal positivism by itself gives no real basis for arguing against the validity of any law based on its particular content, no matter how abhorrent to common morality. The positivist argument is solely about the nature of law as a human institution.
Natural law or the law of nature has been described as a law whose content is set by nature and is thus universal.  As classically used, natural law refers to the use of reason to analyze human nature and deduce binding rules of moral behaviour. The phrase natural law is opposed to the positive law (meaning "man-made law", not "good law"; ) of a given political community, society, or nation-state, and thus can function as a standard by which to criticize that law. In natural law jurisprudence, on the other hand, the content of positive law cannot be known without some reference to the natural law (or something like it). Used in this way, natural law can be invoked to criticize decisions about the statutes, but less so to criticize the law itself. Some use natural law synonymously with natural justice or natural right.

Although natural law is often associated with common law, the two are distinct in that natural law is a view that certain rights or values are inherent in or universally cognizable by virtue of human reason or human nature, while common law is the legal tradition whereby certain rights or values are legally cognizable by virtue of judicial recognition or articulation. Natural law theories have, however, exercised a profound influence on the development of English common law, and have featured greatly in the philosophies of Thomas Aquinas and Thomas Hobbes. Because of the intersection between natural law and natural rights, it has been cited as a component in United States Declaration of Independence. 

In summary, natural law is the idea that there are rational objective limits to the power of legislative rulers. The foundations of law are accessible through human reason and it is from these laws of nature that human created laws gain whatever force they have.
Legal positivism, by contrast to natural law, holds that there is no necessary connection between law and morality and that the force of law comes from some basic social facts although positivists differ on what those facts are.