Just and unjust laws

I would agree with St. Augustine that "An unjust law is no law at all." Now, what is the difference between the two? How does one determine when a law is just or unjust? A just law is a man-made code that squares with the moral law, or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas, an unjust law is a human law that is not rooted in eternal and natural law. Any law that uplifts human personality is just. Any law that degrades human personality is unjust.

- Martin Luther King, Jr., Letter from Birmingham Jail, as cited in The Atlantic