Saints Thomas Aquinas and Augustine taught that an unjust law is no law at all:
“As Augustine says, ‘that which is not just seems to be no law at all‘: wherefore the force of a law depends on the extent of its justice. Now in human affairs, a thing is said to be just, from being right, according to the rule of reason. But the first rule of reason is the law of nature, as is clear from what has been stated above. Consequently, 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 deflects from the law of nature, it is no longer a law but a perversion of law.” Thomas Aquinas, ST I-II, Q. 95, Art. 2.
Since natural law is recognizable to most men, many succeed in conforming their conduct to much of it without the need for punishment. One way of measuring the “justness” of a law is to ask whether most people readily obey a law even without the fear of punishment. By this measure, laws against murder, theft, and assault are “just” laws.
Contrast these laws—which are widely respected—with laws designed to control people who are not harming anyone (except perhaps themselves). These people may be unlicensed barbers, brewers, midwives or street vendors. Or perhaps they are guilty of questionable conduct: gamblers, smokers, drug users & sellers, drunks and adulterers. Most of these people would not kill you even if they could get away with it, but they freely commit these and other offenses, none of which harm anyone but themselves.
Some of these actions are widely considered to be immoral vices, while other conduct is prohibited for various political/regulatory reasons. People are more likely to commit such offenses when the law appears arbitrary, meddlesome and paternalistic; especially if the likelihood of being caught is low.
The test of a legitimate government
All governments control their citizens through violence. Less authoritarian nations rule with less force, while others are massively violent, but the common denominator that defines the modern state is the initiation of violent force. “What essentially sets a nation-state apart,” declared candidate Barack Obama, is that it has a “monopoly on violence.” There is a reason why the state must use force or violence to enforce its decrees, and the Church explains why in the Catechism of the Catholic Church:
“1930. Respect for the human person entails respect for the rights that flow from his dignity as a creature. These rights are prior to society and must be recognized by it. They are the basis of the moral legitimacy of every authority: by flouting them, or refusing to recognize them in its positive legislation, a society undermines its own moral legitimacy. If it does not respect them, authority can rely only on force or violence to obtain obedience from its subjects. . . .” CCC ¶ 1930 (emphasis added).
What are these human rights that must be respected by the state? A good start is the right to life, but in addition to that, such rights include the right to own private property and the right to spend our earnings however we chose. The principal purpose of the state is to protect these rights and the right to personal and collective security; that is, to promote the common good.
The government that “flouts” the rights of people is an unjust government. An unjust government “can rely only on force or violence to obtain obedience from its subjects.” This truth suggests a simple test of authority: to the extent that any state uses aggression to obtain obedience from its subjects, it is illegitimate.