Unlike civil actions, where plaintiffs seek compensation or other remedies for themselves, crimes involve “the state” (the federal government, a state government, or some subunit of state government). This is because crimes involve some “harm to society” and not just harm to certain individuals. But “harm to society” is not always evident in the act itself. For example, two friends of yours at a party argue, take the argument outside, and blows are struck; one has a bloody nose and immediately goes home. The crimes of assault and battery have been committed, even though no one else knows about the fight and the friends later make up. By contrast, suppose a major corporation publicly announces that it is closing operations in your community and moving operations to Southeast Asia. There is plenty of harm to society as the plant closes down and no new jobs take the place of the company’s jobs. Although the effects on society are greater in the second example, only the first example is a crime.

Crimes are generally defined by legislatures, in statutes; the statutes describe in general terms the nature of the conduct they wish to criminalize. For government punishment to be fair, citizens must have clear notice of what is criminally prohibited. Ex post facto laws—laws created “after the fact” to punish an act that was legal at the time—are expressly prohibited by the US Constitution. Overly vague statutes can also be struck down by courts under a constitutional doctrine known as “void for vagueness.”

What is considered a crime will also vary from society to society and from time to time. For example, while cocaine use was legal in the United States at one time, it is now a controlled substance, and unauthorized use is now a crime. Medical marijuana was not legal fifty years ago when its use began to become widespread, and in some states its use or possession was a felony. Now, some states make it legal to use or possess it under some circumstances. In the United States, you can criticize and make jokes about the president of the United States without committing a crime, but in many countries it is a serious criminal act to criticize a public official.

Attitudes about appropriate punishment for crimes will also vary considerably from nation to nation. Uganda has decreed long prison sentences for homosexuals and death to repeat offenders. In Saudi Arabia, the government has proposed to deliberately paralyze a criminal defendant who criminally assaulted someone and unintentionally caused the victim’s paralysis. Limits on punishment are set in the United States through the Constitution’s prohibition on “cruel or unusual punishments.”

It is often said that ignorance of the law is no excuse. But there are far too many criminal laws for anyone to know them all. Also, because most people do not actually read statutes, the question of “criminal intent” comes up right away: if you don’t know that the legislature has made driving without a seat belt fastened a misdemeanor, you cannot have intended to harm society. You might even argue that there is no harm to anyone but yourself!

The usual answer to this is that the phrase “ignorance of the law is no excuse” means that society (through its elected representatives) gets to decide what is harmful to society, not you. Still, you may ask, “Isn’t it my choice whether to take the risk of failing to wear a seat belt? Isn’t this a victimless crime? Where is the harm to society?” A policymaker or social scientist may answer that your injuries, statistically, are generally going to be far greater if you don’t wear one and that your choice may actually impose costs on society. For example, you might not have enough insurance, so that a public hospital will have to take care of your head injuries, injuries that would likely have been avoided by your use of a seat belt.

But, as just noted, it is hard to know the meaning of some criminal laws. Teenagers hanging around the sidewalks on Main Street were sometimes arrested for “loitering.” The constitutional void-for-vagueness doctrine has led the courts to overturn statutes that are not clear. For example, “vagrancy” was long held to be a crime, but US courts began some forty years ago to overturn vagrancy and “suspicious person” statutes on the grounds that they are too vague for people to know what they are being asked not to do.

This requirement that criminal statutes not be vague does not mean that the law always defines crimes in ways that can be easily and clearly understood. Many statutes use terminology developed by the common-law courts. For example, a California statute defines murder as “the unlawful killing of a human being, with malice aforethought.” If no history backed up these words, they would be unconstitutionally vague. But there is a rich history of judicial decisions that provides meaning for much of the arcane language like “malice aforethought” strewn about in the statute books.

Because a crime is an act that the legislature has defined as socially harmful, the parties involved cannot agree among themselves to forget a particular incident, such as a barroom brawl, if the authorities decide to prosecute. This is one of the critical distinctions between criminal and civil law. An assault is both a crime and a tort. The person who was assaulted may choose to forgive his assailant and not to sue him for damages. But he cannot stop the prosecutor from bringing an indictment against the assailant. (However, because of crowded dockets, a victim that declines to press charges may cause a busy prosecutor to choose to not to bring an indictment.)

A crime consists of an act defined as criminal—an actus reus—and the requisite “criminal intent.” Someone who has a burning desire to kill a rival in business or romance and who may actually intend to murder but does not act on his desire has not committed a crime. He may have a “guilty mind”—the translation of the Latin phrase mens rea—but he is guilty of no crime. A person who is forced to commit a crime at gunpoint is not guilty of a crime, because although there was an act defined as criminal—an actus reus—there was no criminal intent.