Morissette v. United States is a case that dealt with the interpretation of a statute that criminalized the conversion of government property. The Supreme Court held that the government must prove that the defendant knew the property belonged to the government and intended to convert it. The lower court erred in instructing the jury that the defendant's ignorance of the property's ownership was not a defense. The Court believes that a harmful act must be intentional to be considered a crime, and that there must be a mental element involved in the commission of a crime. American law requires both an evil intent and an evil act for a crime. However, there are "public welfare offenses" that do not require intent, only forbidden acts or omissions, due to new duties and crimes created to protect public health, safety, and welfare. The Court has aligned with state judicial opinion on federal legislation, which may have led to a lack of explanation of their reasoning. The Court also determined that intent is not necessary for an offense under the Narcotic Drug Act. This type of legislation uses penalties as a means of regulation and puts the burden of acting responsibly on those in a position to prevent public danger, even if they are otherwise innocent.

The court has not established clear criteria for distinguishing between crimes that require a mental element and those that do not. Eliminating the requirement of a guilty intent in common-law crimes would harm the defendant's rights and limit the freedom of juries. The federal judiciary should not create crimes or expand their reach beyond what is specified in the statute. The statute only criminalizes "knowing" conversions, but common law recognizes unwitting acts as conversions. The trial court erred in deciding the presence of intent as a presumption of law based on the isolated act of taking. The existence of intent is a question of fact that must be submitted to the jury. The conclusion of intent to steal based solely on the fact that the defendant took the property is not adequate. The case is reversed, and two justices concur in the decision.