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Let us first be clear that by "natural law" we mean principles of human conduct, not the laws of nature discovered by the physical sciences. Many thinkers who espouse natural law see it at work in both the human and nonhuman realms, but their main interest is in its special application to man. According to these thinkers, the natural law as applied to physical things or animals is inviolable; stars and atoms never disobey the laws of their nature. But man often violates the moral rules which constitute the law of his specifically human nature.

The idea of a natural right order to which all things, including human beings, should conform is one of the most ancient and universal notions. It is a major principle in the religious and philosophic systems of ancient India and China, as well as in classical Greek philosophy. Plato calls it "justice" and applies it to the human soul and human conduct.

In Western society, especially from the Roman jurists and the theologians of the Middle Age on, we find the doctrine of the natural moral law for man. It is the source of moral standards, the basis of moral judgments, and the measure of justice in the man-made laws of the state. If the law of the state runs counter to the precepts of the natural law, it is held to be unjust.

The first precept of natural law is to seek the good and avoid evil. It is often put as follows: "Do good unto others, injure no one, render to every man his own." Now, of course, such a general principle is useless for organized society unless we can use it to specify various types of rights and wrongs. That is precisely what man-made, or positive, law tries to do.

Thus, the natural law tells us only that stealing is wrong because it inflicts injury, but the positive law of larceny defines the various kinds and degrees of theft and prescribes the punishments therefor.

Such particular determinations may differ in various times and places without affecting the principles of natural law. Neither Aquinas nor Aristotle thinks that particular rules of laws should be the same in different times, places, and conditions.

You may ask how the natural law is known. Through human reason and conscience, answer the natural-law thinkers. The natural-law doctrine usually assumes that man has a specific nature which involves certain natural needs, and the power of reason to recognize what is really good for man in terms of these needs.

Christian thinkers, such as Aquinas and John Locke, think the natural law is of divine origin. God, in creating each thing, implanted in it the law of its nature. The phrase about "the laws of nature and of nature's God" in our Declaration of Independence derives from this type of natural-law doctrine. However, this particular theological viewpoint is not always found in writers who uphold the natural law, for these include such pre-Christian thinkers as Plato, Aristotle, and Cicero, and such modern secular philosophers as Kant and Hegel.

There has been much opposition to natural-law philosophy from the very beginning. Indeed, one might say the opposition came first, for the idea of natural right or justice was developed in ancient Greece to counter the views of the Sophists, who were "conventionalists." These men believe that law and justice are simply man-made conventions. No action is right or wrong unless a particular community, through its positive laws or customs, decrees that it is right or wrong. Then it is right or wrong in that particular place and time -- not universally. By nature, the Sophists say, fire burns in Greece as it does in Persia, but the laws of Persia and of Greece, being matters of convention, are not the same. The "conventionalist" or "positivist" doctrine of law has come down all the way from the ancient Sophists to many of our modern law-school professors.

You ask whether natural law is relevant to modern conditions. My answer is that if justice is still relevant, then natural law is. Indeed, interest in natural law has increased especially during the past half century, with its experience of the kind of positive laws which have been imposed by totalitarian regimes. On what grounds could a decent German citizen in Nazi times justify his opposition to the laws of the land? On private sentiments or merely personal opinion? Even purely inner resistance to iniquity must be rooted in firmer grounds. "A law which is not just is a law in name only," says Augustine. And Aquinas adds: "Every human law has just so much of the nature of law as it is derived from the law of nature. But if in any point it departs from the law of nature, it is no longer a law but a perversion of the law."

The naturalists, as that name indicates, affirm the existence of natural justice, of natural and unalienable rights, of the natural moral law, and of valid prescriptive oughts that elicit our assent, both independently of and prior to the existence of positive law. The positivists deny all this and affirm the opposite. For them, the positive law -- the man-made law of the state -- provides the only prescriptive oughts that human beings are compelled to obey. According to them, nothing is just or unjust until it has been declared so by a command or prohibition of positive law.

If this is a fundamentally erroneous view, as I think it is, its ultimate roots lie very deep. They rise from the most profound mistake that can be made in our thinking about good and evil. It is the mistake made by those who embrace an unattenuated subjectivism and relativism with respect to what is good and bad, right and wrong.

Neglecting or rejecting the distinction between real and apparent goods, together with that between natural needs and acquired wants, the positivists can find no basis for the distinction between what "ought" to be desired or done and what is desired or done. From that flows the further consequence that there is no natural moral law, no natural rights, no natural justice, ending up with the conclusion that man-made law alone determines what is just and unjust, right and wrong.

This positivist view is as ancient as the despotisms that existed in antiquity. It was first eloquently expressed in the opening book of Plato's "Republic" where Thrasymachus, responding to Socrates' mention of the view that justice consists in rendering what is due, declared and defended the opposite view -- that justice is the interest of the stronger. Spelled out, this means that what is just or unjust is determined solely by whoever has the power to lay down the law of the land.

The positivist view is recurrent in later centuries with the recurrence of later despotisms. It was expressed by the Roman jurisconsult, Ulpian, who, defending the absolutism of the Caesars, declared that whatever pleases the prince has the force of law. Still later, in the sixteenth century, the same view was set forth by another defender of absolute government, Thomas Hobbes, in "The Leviathan"; and later, in the nineteenth century, by John Austin, in his "Analytical Jurisprudence."

Neither Austin nor the twentieth-century legal positivists who follow him regard themselves as defenders of absolute government or despotism. That is what they are, however -- perhaps not as explicitly as their predecessors, but by implication at least. The denial of natural rights, the natural moral law, and natural justice leads not only to the positivist conclusion that man made law alone determines what is just and unjust. It also leads to a corollary which inexorably attaches itself to that conclusion -- "that might makes right" -- this is the very essence of absolute or despotic government.