Writing for the Court, Justice Robert Jackson criticized the trial court’s jury instructions to presume Morissette’s felonious intent merely by his taking the bomb casings, and also rejected the Sixth Circuit’s conclusion that 18 U.S.C. §641 required no element of criminal intent because Congress didn’t explicitly include one. Jackson also warned that reliance on cases such as United States v. Balint (1922) and United States v. Behrman (1922), if applied literally to all federal criminal statutes, would sweep away the fundamental requirement of culpability.

Jackson explained the historical and philosophical importance of mens rea, emphasizing that the idea that an injury can amount to a crime only when inflicted by intention is a universal and persistent one in “mature” systems of law. Jackson traced this principle to Blackstone’s requirement of a “vicious will”, and noted that crime is generally constituted only from the concurrence of an “evil-meaning mind with an evil-doing hand.” Jackson further explained that at the time of the founding, states codified the crimes found in common law, and courts consistently assumed that even if a statute was silent on the subject of intent, the requirement was so inherent in the idea of the offense that it didn’t need to be explicitly stated..

Jackson then distinguished traditional common-law crimes from modern “public welfare offenses,” which fall into a different category. Jackson explained that the industrial revolution brought about complex mechanisms, congested cities, and wide distribution of goods, creating intolerable casualty risks to public health, safety, and welfare. To enforce new duties and standards of care, lawmakers created regulations where the injury is the same regardless of the violator’s intent. In these public welfare offenses, which often involve neglect or inaction rather than positive action, carry relatively small penalties, and do not seriously damage an offender’s reputation. In such cases, courts have construed statutory silence as dispensing with the intent requirement, holding that the guilty act alone makes out the crime.

Jackson, however, made it clear that stealing, larceny, and its variants do not belong in the category of public welfare offenses. These crimes are ancient, common-law invasions of property rights that carry high penalties and the infamy of a felony conviction. As a result, Jackson established a rule of statutory construction, explaining when Congress borrows “terms of art” in which are accumulated the legal tradition and meaning of centuries of practice (such as "steals" or "converts"), they presumably know and adopt the cluster of ideas that were attached to each borrowed word. Jackson affirmed that the absence of any other direction from Congress must be taken as satisfaction with widely accepted definitions, not as a departure from them.

Ultimately, the Court held that the mere omission of any mention of intent from the statute will not be construed as eliminating that element from the crimes denounced. Jackson explained that by presuming Morissette’s intent from his actions and refusing to allow the jury to consider whether he acted with an innocent belief that the property was abandoned, the trial court changed the balance in the scales of justice and unconstitutionally circumscribed the freedom of the jury. Therefore, the conviction could not stand.