Raymond B. answered 12/31/20
Math, microeconomics or criminal justice
It's similar to "natural law" or the closest we've come to it. As Martin Luther King said, if it's an unjust law, it's not a legitimate law. "natural law" is what's just and fair.
Humans are imperfect, but with the common law, they came closest. It's judge made law, crimes are the same as torts, requiring a real victim. But then came legislatures. Give them the power to abuse, and they abuse it. Power corrupts and absolute power corrupts absolutely, as Lord Acton famously said.
Judges at least restricted themselves to what seemed fair and just. Legislatures began making endless crimes, so many no one knows but a tiny fraction of what Congress or state legislatures have made illegal. And ignorance of the law is no excuse, even if you're prosecuted for a "crime" that even the prosecutor, judge and jury never heard of before.
Common law is what's common among diverse judges. They find a legal conclusion, that's sort of a consensus across diverse judges. America got it from England. England is famous for the Magna Carta.
Today, with legislative law replacing common law, all in the supposed virtue of advance notice of written law over natural law, everyone is an easy target for any prosecutor who wants to put you in prison.
Still common law is so persuasive, much written legislative law has to track it, to some extent. When it deviates, that's the major injustice. That's the purpose or goal of legislatures in passing bills to make endless things illegal (when there is no victim) to please lobbyists and to make congress people act like they're "tough on crime"
It's law up on an auction block, with lobbyists buying the worst laws if they bid high enough. Law is now for the highest bidder, the wealthy, not the poor. It's an upside down world.
The opposite of natural law, just law is the Hobbes world of positive law, where nothing is recognized as law except what the King or government authority says is law. Common law puts law in the hands of the judiciary, and out of the legislature. It's not a perfect or even close to perfect system, it's just better than the legislative "law" Hobbes saw the world as short, nasty and brutish, in need of a totalitarian dictator, with no room for rights or freedom. Common law sides with our Bill of Rights and liberty. Legislative written law is on Hobbes' side.