The nature of a thing is seen in what it does.  The nature of a bird is seen in its flight.  The nature of man is seen in action.  Action reveals intention.  Intention reveals the structure of ethics.  Natural law is not invented.  It’s discovered.  It rests on the simple fact that man acts.  He acts to satisfy wants.  He evaluates means.  He prefers one state of affairs to another.  This is the root of reason.

There is widespread confusion about natural law.  That doesn’t mean the law is unclear.  It means people are confused, intentionally or unintentionally.  The natural law is the principle that each individual owns himself.  His body.  His mind.  His labor.  Whatever his labor transforms becomes his property.  Locke identified this cleanly.  “Every man has a property in his own person.” From this, the boundary of justice is drawn.  Violence is the uninvited crossing of that boundary.

Ethics refers to the legality of actions in relation to property.  Morals are what a given community considers right or wrong.  Morals differ between cultures.  Ethics does not.  A society may celebrate self-sacrifice or condemn it.  That is moral taste.  No society can ethically justify stealing, even if it calls the theft charity, taxation, or necessity.  Legality and acceptance are separate things.  Something can be legal and still be unjust.  Something can be socially praised and still be violent.

Natural law is value free.  It doesn’t say what ends to pursue.  It only identifies the boundary that can’t be crossed without committing violence.  When someone advocates a policy, he has already made a value judgment.  He prefers a world where his will is enforced upon others.  Understanding natural law reveals this.  It exposes where violence is being smuggled in under polite language.

Now consider positive law.  Positive law is created by man.  It’s written, legislated, decreed, and enforced.  It’s power with a mask.  If Crusoe catches fish and Friday gathers berries, the fish belong to Crusoe and the berries belong to Friday.  Both mixed labor with an unowned resource.  Property is clear.  If Friday demands Crusoe give him fish against his will, that’s aggression.  If Friday writes down a rule that states ‘Crusoe must give up fish,’ that’s positive law.  Nothing changed about the nature of the act.  The paper only masks it.

Natural law must be discovered.  Positive law must be declared.  Natural law stands whether it’s recognized or ignored.  Positive law exists only so long as force maintains it.

One reveals justice.
The other restricts it.

Reference

Murray Rothbard; The Ethics of Liberty