Introduction
Criminal Law begins with a deceptively simple question: What exactly did the defendant do that the law permits the state to punish?
A harmful result alone ordinarily does not answer that question. A person may cause harm accidentally, may possess a bad character without committing a crime, may think about criminal conduct without acting, or may stand by while another person suffers without having any legal obligation to intervene. Criminal liability therefore requires a structured analysis connecting a legally prohibited form of conduct to the other elements of the offense.
At its most basic level, a criminal offense ordinarily requires:
- A prohibited act, omission, or qualifying possession (Actus Reus).
- The required mental state (Mens Rea).
- Concurrence between the required conduct and mental state.
- Causation when the offense requires a prohibited result.
Each component serves a different purpose.
The external conduct requirement is commonly called actus reus. It prevents criminal punishment from resting solely on thoughts, status, personality, or moral disapproval.
The mental-state requirement, often called mens rea, asks what level of culpability the defendant must possess concerning the relevant elements.
Concurrence asks whether the required mental state existed in connection with the legally significant conduct.
Causation becomes important when the offense requires the defendant to cause a prohibited result, such as death, bodily injury, or property destruction.
Before reaching any of those issues, however, criminal law asks an even more basic question: Was the conduct criminal when the defendant acted? That is the principle of legality.
I The Principle of Legality
The principle of legality provides that conduct cannot ordinarily be punished as a crime unless the law prohibited that conduct before the defendant acted.
The core idea is straightforward: Government should define criminal conduct before imposing criminal punishment for it.
Preexisting Criminal Law
A person ordinarily may not be convicted merely because conduct appears immoral or dangerous. There must be a valid legal prohibition applicable when the conduct occurred. This rejects retroactive criminal punishment.
Fair Notice
Criminal laws should be written with sufficient clarity that ordinary people can understand what conduct is prohibited. If a law is too vague ("behave badly in public"), it fails to give notice.
Restrictions on Vagueness
Vagueness doctrine ensures people have notice AND provides meaningful standards limiting arbitrary or discriminatory enforcement by police.
Ambiguous Penal Statutes (Rule of Lenity)
When a criminal statute remains genuinely ambiguous after ordinary methods of interpretation have been exhausted, courts may construe the ambiguity narrowly in favor of the defendant.
II Statutory Interpretation
Modern Criminal Law is heavily statutory. Students should therefore develop the habit of converting every statute into an element checklist before applying facts.
Types of Elements
Conduct Elements
Identifies what the defendant must do (e.g., Enter, Take, Possess, Strike, Sell).
Result Elements
Requires a particular consequence (e.g., Causing death, Causing bodily injury). Causation becomes part of the analysis.
Attendant Circumstances
Conditions surrounding the conduct/result that must be present (e.g., Property belongs to another, At night, Lacks permission).
Mens Rea Terms
Mental-state language defining culpability (e.g., Intentionally, Knowingly, Recklessly).
Parsing Example: Aggravated Burglary
"A person commits aggravated burglary if the person knowingly enters the dwelling of another without permission, at night, with intent to commit a felony therein."
Do not summarize as:
"Burglary is entering someone's house to commit a crime."
Break it apart into elements:
- The defendant entered.
- The entry was knowing.
- The place was a dwelling.
- The dwelling belonged to another.
- The defendant lacked permission.
- The conduct occurred at night.
- The defendant possessed intent to commit a felony inside.
If even one required element is missing, the charged statutory offense may fail.
III Actus Reus & Voluntary Conduct
Actus reus refers to the legally required external conduct. The law ordinarily requires something that can be attributed to the defendant in the external world. That requirement may take the form of a voluntary act, a legally sufficient omission, or possession.
Why Thoughts Alone Are Not Enough
Suppose D intensely dislikes V and privately imagines killing V but takes no step toward doing so. Criminal law ordinarily does not punish uncommunicated thoughts by themselves. This protects individual autonomy and limits government punishment to conduct rather than internal mental life.
Voluntary vs. Involuntary Acts
A voluntary act is generally a bodily movement resulting from conscious choice (e.g., walking, driving, pulling a trigger). The defendant does not need to have made a wise or calm choice. The question is narrower: Was the bodily movement attributable to conscious action?
Examples that may NOT qualify as voluntary acts include:
- Reflexes.
- Convulsions.
- Bodily movement during unconsciousness.
- Movement during certain sleep states.
- Conduct occurring while physically forced by another.
Exam Tip: Voluntariness is not Motive
Suppose D drives because D’s employer threatens to fire D unless D reports to work. D feels intense economic pressure, but D's operation of the vehicle is still a voluntary bodily act. The existence of pressure does not automatically make the physical movement involuntary.
Time-Framing
A criminal offense does not necessarily require every movement in the chain of events to be voluntary. It may be sufficient that criminal liability can be tied to a voluntary act occurring at the legally relevant point in the course of conduct.
The Seizure Example
Suppose D knows that D suffers unpredictable seizures but nevertheless voluntarily drives. While driving, D loses consciousness because of a seizure, crosses the center line, and causes a collision.
The precise movement of the steering wheel during unconsciousness may be involuntary. However, the analysis should not end there. The earlier decision to drive was voluntary. If the governing offense permits liability to be tied to that earlier conduct (e.g., reckless driving), actus reus is satisfied.
IV Omissions & Legal Duty
Criminal law generally does not impose liability merely because a person failed to help another. A failure to act ordinarily becomes criminally significant only when the defendant had a legal duty to act. A moral duty is not enough.
The Omission Framework (4 Questions):
- Was there a legal duty to act?
- Was the defendant physically capable of performing the required act?
- Did the omission cause the prohibited result (when causation is required)?
- Did the defendant possess the required mens rea?
Sources of a Legal Duty to Act
1. Statute
A law expressly requires action (e.g., filing tax returns, reporting accidents).
2. Contract
A contractual undertaking creates responsibility (e.g., a hired lifeguard, a paid caretaker).
3. Special Relationship
Relationships involving dependency (e.g., parent/child, spouses, jailer/prisoner).
4. Voluntary Assumption of Care
Starting to help and leaving the victim dependent or isolating them from other aid.
5. Creation of Risk
Creating a dangerous condition (even innocently) generates a duty to take reasonable steps to prevent resulting harm (e.g., accidentally knocking someone into a pool).
Common Trap
Do not write, "D could have saved V, so D is criminally liable." Ability to help is not enough. You must identify the specific legal duty before an omission can be an actus reus.
V Possession
Possession offenses demonstrate that actus reus does not always require obvious physical movement at the moment of arrest.
Actual Possession
Ordinarily means direct physical custody or control. Examples include contraband in D's hand, in D's pocket, or an object D is personally carrying.
Constructive Possession
Permits possession to be established even if the item is not physically on the defendant. Requires power to exercise dominion/control, intent/ability to exercise it, and knowledge of the item.
Mere Presence vs Control
One of the most common possession mistakes is to equate physical closeness with possession. If police discover contraband under a chair in a crowded living room, the fact that D is standing nearby is mere proximity. It does not necessarily prove D possessed the item without evidence of dominion, control, and knowledge.
Timing and Unknowing Receipt
Possession can satisfy the actus-reus requirement when the defendant knowingly obtains the item OR retains control after becoming aware of the item and has sufficient time to terminate possession.
If a friend secretly hides a bag in D's apartment, D does not culpably possess it. If D later discovers it, recognizes it as illegal, and hides it in a closet for a week, that continued control establishes possession.
VI. Status Offenses
The external-conduct requirement also limits punishment based solely on status. Criminal law generally distinguishes between being a certain kind of person and doing prohibited conduct.
For example, punishing prohibited drug use (conduct) is conceptually different from punishing a person merely because they have the condition of addiction (status). A state may disapprove of a person's condition or reputation, but criminal punishment ordinarily must connect to prohibited conduct.
VII. Integrated Hypothetical
"D knows D suffers recurring episodes that can cause sudden unconsciousness. A physician has warned D not to drive. D nevertheless voluntarily drives to a friend’s apartment. At the apartment, Friend secretly places a bag containing illegal contraband in D’s vehicle. D drives away without knowing about the bag. Ten minutes later, D discovers the bag, recognizes the contraband, but decides to keep driving to return it to Friend later. D then sees a child struggling in a pond. D has no relationship with the child and didn't create the danger. D drives away. Minutes later D loses consciousness, the vehicle crashes, and a pedestrian is injured."
1. The Crash (Actus Reus/Time-Framing): The movement during unconsciousness was involuntary. However, D's earlier decision to drive despite known episodes supplies legally attributable voluntary conduct.
2. The Contraband (Possession): D did not knowingly obtain the contraband initially. But once D discovered it and continued exercising control over the vehicle containing it (choosing to drive it back later), D knowingly retained constructive or actual possession.
3. The Child (Omissions): D's failure to rescue is morally troubling but does not create criminal omission liability. D had no special relationship, statutory duty, or contractual duty, and did not create the danger. Mere ability to call for help does not establish a legal duty.
Chapter Summary
Criminal liability ordinarily requires more than a harmful result. The basic architecture includes prohibited conduct (Actus Reus), the required mental state (Mens Rea), concurrence, and causation when the offense requires a prohibited result.
The principle of legality requires criminal prohibitions to exist before punishment is imposed. It supports fair notice, limits vague laws, and restricts retroactive punishment. Because modern criminal law is statutory, students must parse every offense into its components: conduct, results, attendant circumstances, and mens rea terms.
Actus reus refers to the legally required external conduct. The law does not punish thoughts or status alone. The requirement may be satisfied by a voluntary act, a legally sufficient omission, or qualifying possession.
A voluntary act is a bodily movement resulting from conscious choice. Reflexes, convulsions, and movements during unconsciousness are involuntary. However, through time-framing, liability can often be tied to an earlier voluntary act (e.g., choosing to drive knowing you have seizures).
Omission liability requires a legal duty to act, not just a moral duty. Sources of legal duty include statutes, contracts, special relationships, voluntary assumption of care, and creation of the risk.
Possession can satisfy actus reus. Actual possession is direct physical custody. Constructive possession requires knowledge of the item and the power/intent to exercise dominion and control. Mere proximity is insufficient.
The key lesson for Monday is foundational: Criminal liability begins with legally attributable conduct.