It seems commonsensical that for criminal punishment to be just and legal, the activity punished must have been made illegal.

However, legality is a more complex subject than it seems, as the cases below illustrate. Legislatures and courts struggle to define and interpret criminal law, and the roles and relationships between these institutions in determining what is criminal have evolved over time. Consider the strengths and weaknesses, advantages and disadvantages of courts and of legislatures in defining particular crimes — a topic you have undoubtedly encountered throughout your 1L year.

Additionally, giving people notice of criminal proscription underpins the idea of legality. Consider the issue of notice. Given the limited knowledge that most people have of the law, can they be said to have actual notice of what conduct is criminal? And should this matter? As you will see, courts sometimes invalidate convictions due to lack of notice, such as when a statute is unconstitutionally vague. Given that most people don’t read criminal statutes, why do courts go to such lengths to uphold the principle of notice?

Actus reus, or the act requirement, is the first part of culpability in criminal law. (You will meet the second part, mens rea, in the next section.) In short, almost every crime must have an act, but defining that act can be tricky. Sometimes something that seems like an act isn’t an act; other times, something that does not seem like an act is one.

These cases introduce you to the act requirement. Notice distinctions between voluntary and involuntary acts, and between conduct and the results of conduct. Consider why the court reaches the decision it does in each case, and what its decision says about its concept of blameworthiness.

Mens rea—a guilty mind—is the second part of criminal culpability, and undoubtedly one of the most complex subjects we will study in this course. Intricately tied into concepts of blameworthiness, mens rea can determine whether the same conduct and result constitute a blameless accident or a capital offense.

The cases and readings in this section represent a range of mens rea categories, from a lack of mens rea to various grades of mens rea: negligence, recklessness, knowledge, and purpose. As you will see here and throughout this course, there are gradations and exceptions even within these categories.

The questions these cases raise are fundamental to the study of criminal law. As you read through them, consider why each crime requires the mens rea that is attached to it, whether you think that requirement is fair, and the impact of the mens rea requirement on the enforcement of the law. How would the crime have been adjudicated under different mens rea requirements? Does the requirement track your sense of moral blameworthiness?