Crimes are divided into two classes: those that are wrong in and of themselves such as murder, rape, and robbery, and those that are not in themselves wrong but are criminal simply because they are prohibited by statute.

The former are called crimes mala in se and require a wrongful or unlawful intent on the part of the perpetrator.

The latter are called crimes mala prohibita and require no wrongful intent on the part of the perpetrator. All that is necessary is the doing of the act regardless of the intent of the actor.

Under the common law that is, the statutory and case law used in England and the American colonies before the American revolution, all crimes were mala in se.

To illustrate a crime mala prohibita, a 1906 state statute made it a crime to transport intoxicating liquor within the state without a license.

A truck driver in the employ of a common carrier was convicted of violating the statute when he transported an unmarked sugar barrel filled with liquor.

Nothing about the appearance of the barrel caused suspicion as to its contents, and the truck driver was ignorant of the fact that it contained intoxicating liquor.

The appellate court upheld the conviction, saying that the only fact to be determined is whether the defendant did the act.

The court held that knowledge of the wrongdoing or wrongful intent was immaterial in the case of a crime mala prohibita. The court said that the legislature has the power to prohibit certain acts regardless of moral purity or ignorance.