In his "Letter from a Birmingham Jail," Martin Luther King says that he believes, like St. Augustine, that an unjust law is no law at all. As one has the duty to obey just laws, so one has the duty to disobey unjust laws. He then gives a definition:
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 law and natural law.
King later gives examples of unjust laws, pointing out that "it was 'illegal' to aid and comfort a Jew in Hitler's Germany." He also points out that a law which appears just on its face can be applied unjustly.
There are many unjust laws around the world, the most obvious being those that discriminate between the rights given to different groups. In the United States, the most egregious examples tend to be on a state rather than on a federal level. Examples include laws denying equal treatment to homosexuals.
However, there is at least one federal law...
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