Natural law theories have exercised a profound influence on the development of English common law, and have featured greatly in the philosophies of Thomas Aquinas, Francisco Suárez, Richard Hooker, Thomas Hobbes, Hugo Grotius, Samuel von Pufendorf, and John Locke. Because of the intersection between natural law and natural rights, it has been cited as a component in United States Declaration of Independence.
Aristotle's association with natural law is due largely to the interpretation given to his works by Thomas Aquinas. This was based on Aquinas's conflation of natural law and natural right, the latter of which Aristotle posits in Book V of the Nicomachean Ethics (= Book IV of the Eudemian Ethics). Aquinas's influence was such as to affect a number of early translations of these passages, though more recent translations render them more literally. Aristotle notes that natural justice is a species of political justice, viz. the scheme of distributive and corrective justice that would be established under the best political community; were this to take the form of law, this could be called a natural law, though Aristotle does not discuss this and suggests in the Politics that the best regime may not rule by law at all.
The best evidence of Aristotle's having thought there was a natural law comes from the Rhetoric, where Aristotle notes that, aside from the "particular" laws that each people has set up for itself, there is a "common" law that is according to nature. The context of this remark, however, suggests only that Aristotle advised that it could be rhetorically advantageous to appeal to such a law, especially when the "particular" law of one's own city was averse to the case being made, not that there actually was such a law; Aristotle, moreover, considered two of the three candidates for a universally valid, natural law provided in this passage to be wrong. Aristotle's theoretical paternity of the natural law tradition is consequently disputed.
All human laws were to be judged by their conformity to the natural law. An unjust law was in a sense no law at all. At this point, the natural law was not only used to pass judgment on the moral worth of various laws, but also to determine what the law said in the first place. This could result in some tension.
The natural law was inherently deontological in that although it is aimed at goodness, it is entirely focused on the ethicalness of actions, rather than the consequence. The specific content of the natural law was therefore determined by a conception of what things constituted happiness, be they temporal satisfaction or salvation. The state, in being bound by the natural law, was conceived as an institution directed at bringing its subjects to true happiness. In the 16th century, the School of Salamanca (Francisco Suárez, Francisco de Vitoria, etc.) further developed a philosophy of natural law. After the Church of England broke from Rome, the English theologian Richard Hooker adapted Thomistic notions of natural law to Anglicanism.
The concept of Istislah in Islamic law bears some similarities to the natural law tradition in the West, as exemplified by Thomas Aquinas. However, whereas natural law deems good that which is known self-evidently to be good, according as it tends towards the fulfilment of the person, istislah calls good whatever is connected to one of five "basic goods". Al-Ghazali abstracted these "basic goods" from the legal precepts in the Qur'an and Sunnah: they are religion, life, reason, lineage and property. Some add also "honour".
Ibn Qayyim al-Jawzi also posited that human reason could discern between 'great sins' and good deeds.
Abū Rayhān al-Bīrūnī, an Islamic scholar and polymath scientist, understood natural law as the law of the jungle. He argued that the antagonism between human beings can only be overcome through a divine law, which he believed to have been sent through prophets. This is also the position of the Ashari school the largest school of Sunni theology.
Averroes in his treatise on "Justice and Jihad," and his commentary on Plato's Republic writes that the human mind can know of the unlawfulness of killing and stealing and thus of the five maqasid or higher intents of the Islamic sharia or to protect religion, life, property, offspring, and reason.
As used by Thomas Hobbes in his treatises Leviathan and De Cive, natural law is "a precept, or general rule, found out by reason, by which a man is forbidden to do that which is destructive of his life, or takes away the means of preserving the same; and to omit that by which he thinks it may best be preserved."
According to Hobbes, there are nineteen Laws. The first two are expounded in chapter XIV of Leviathan("of the first and second natural laws; and of contracts"); the others in chapter XV ("of other laws of nature").
Hugo Grotius based his philosophy of international law on natural law. In particular, his writings on freedom of the seas and just war theory directly appealed to natural law. About natural law itself, he wrote that "even the will of an omnipotent being cannot change or abrogate" natural law, which "would maintain its objective validity even if we should assume the impossible, that there is no God or that he does not care for human affairs." (De iure belli ac pacis, Prolegomeni XI). This is the famous argument etiamsi daremus (non esse Deum), that made natural law no longer dependent on theology.
John Locke incorporated natural law into many of his theories and philosophy, especially in Two Treatises of Government. There is considerable debate about whether his conception of natural law was more akin to that of Aquinas (filtered through Richard Hooker) or Hobbes' radical reinterpretation, though the effect of Locke's understanding is usually phrased in terms of a revision of Hobbes upon Hobbesean contractualist grounds. Locke turned Hobbes' prescription around, saying that if the ruler went against natural law and failed to protect "life, liberty, and property," people could justifiably overthrow the existing state and create a new one.
While Locke spoke in the language of natural law, the content of this law was by and large protective of natural rights, and it was this language that later liberal thinkers preferred. Thomas Jefferson, echoing Locke, appealed to unalienable rights in the Declaration of Independence, "We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness."
The Belgian philosopher of law Frank van Dun is one among those who are elaborating a secular conception of natural law in the liberal tradition.
The Church understands human beings to consist of body and mind, the physical and the non-physical (or soul perhaps), and that the two are inextricably linked. Humans are capable of discerning the difference between good and evil because they have a conscience. There are many manifestations of the good that we can pursue. Some, like procreation, are common to other animals, while others, like the pursuit of truth, are inclinations peculiar to the capacities of human beings.
To know what is right, one must use one's reason and apply it to Aquinas' precepts. This reason is believed to be embodied, in its most abstract form, in the concept of a primary precept: "Good is to be sought, evil avoided. St. Thomas explains that:
there belongs to the natural law, first, certain most general precepts, that are known to all;
and secondly, certain secondary and more detailed precepts, which are, as it were,
conclusions following closely from first principles. As to those general principles, the
natural law, in the abstract, can nowise be blotted out from men's hearts. But it is blotted
out in the case of a particular action, insofar as reason is hindered from applying the
general principle to a particular point of practice, on account of concupiscence or some
other passion, as stated above (77, 2). But as to the other, i.e., the secondary precepts, the
natural law can be blotted out from the human heart, either by evil persuasions, just as in
speculative matters errors occur in respect of necessary conclusions; or by vicious customs
and corrupt habits, as among some men, theft, and even unnatural vices, as the Apostle
states (Rm. i), were not esteemed sinful.
However, while the primary and immediate precepts cannot be "blotted out", the secondary precepts can be. Therefore, for a deontological ethical theory they are open to a surprisingly large amount of interpretation and flexibility. Any rule that helps man to live up to the primary or subsidiary precepts can be a secondary precept, for example:
Natural moral law is concerned with both exterior and interior acts, also known as action and motive. Simply doing the right thing is not enough; to be truly moral one's motive must be right as well. For example, helping an old lady across the road (good exterior act) to impress someone (bad interior act) is wrong. However, good intentions don’t always lead to good actions. The motive must coincide with Aquinas's cardinal or theological virtues. Cardinal virtues are acquired through reason applied to nature; they are:
His theological virtues are:
According to Aquinas, to lack any of these virtues is to lack the ability to make a moral choice. For example, consider a man who possesses the virtues of justice, prudence, and fortitude, yet lacks temperance. Due to his lack of self control and desire for pleasure, despite his good intentions, he will find himself swaying from the moral path.
Whereas legal positivism would say that a law can be unjust without it being any less a law, a natural law jurisprudence would say that there is something legally deficient about an unjust law. Legal interpretivism, famously defended in the English speaking world by Ronald Dworkin, claims to have a position different from both natural law and positivism.
The concept of natural law was very important in the development of the English common law. In the struggles between Parliament and the monarch, Parliament often made reference to the Fundamental Laws of England which were at times said to embody natural law principles since time immemorial and set limits on the power of the monarchy. According to William Blackstone, however, natural law might be useful in determining the content of the common law and in deciding cases of equity, but was not itself identical with the laws of England. Nonetheless, the implication of natural law in the common law tradition has meant that the great opponents of natural law and advocates of legal positivism, like Jeremy Bentham have also been staunch critics of the common law.
Natural law jurisprudence is currently undergoing a period of reformulation (as is legal positivism). The most prominent contemporary natural law jurist, Australian John Finnis, is based in Oxford, but there are also Americans Germain Grisez, Robert P. George, and Canadian Joseph Boyle. All have tried to construct a new version of natural law. The 19th-century anarchist and legal theorist, Lysander Spooner, was also a figure in the expression of modern natural law.
"New Natural Law" as it is sometimes called, originated with Grisez. It focuses on "basic human goods," such as human life, knowledge, and aesthetic experience, which are self-evidently and intrinsically worthwhile, and states that these goods reveal themselves as being incommensurable with one another.