Introduction

Mistake and Mental State

Introduction

Two maxims shape most first impressions of this subject, and each is only partly right. "Ignorance of the law is no excuse" sounds like a flat rule that a defendant's wrong belief about the law never matters, and "mistake of fact is an excuse" sounds like a flat rule that a wrong belief about the world always does. Yet a defendant who is mistaken about a fact the offense does not care about gains nothing from the second maxim, while a defendant who is mistaken about a legal matter that forms part of the offense's definition can gain a great deal despite the first. Model Penal Code § 2.04(1) sorts out when a wrong belief helps and when it does not, by providing that ignorance or mistake as to a matter of fact or law is a defense if one of two conditions is met.¹

The idea that organizes the provision is that a mistake matters only through the mental state the offense requires. Every offense defines some prohibited conduct, circumstance, or result and attaches a mental state to it, such as purpose or knowledge. That guilty-mind requirement is what lawyers call mens rea. A wrong belief becomes legally important when it prevents the prosecution from proving the required mental state, and it carries no weight when the mental state is proven despite it. For that reason mistake is the flip side of mens rea: whether an ignorance or mistaken belief is a defense can usually be answered once you know what mental state the statute requires.² Penal codes built on the common law approach the topic through two classifications, the specific-intent and general-intent distinction and the line between mistakes of fact and mistakes of law. The Model Penal Code drops the first of these and asks what function the mistake performs rather than what label it carries.³

The two conditions in § 2.04(1) are two routes to a defense. Under route (a), the ignorance or mistake negatives the purpose, knowledge, belief, recklessness, or negligence required to establish a material element of the offense.⁴ This route does most of the work, because it applies the offense's own mental-state requirement to what the defendant actually believed. Route (b) is narrower: the mistake is a defense when the law provides that the state of mind established by the mistake constitutes a defense, so the defense then rests on a separate rule of law and not on the elements of the offense.⁵ The rest of § 2.04 either limits these routes or asks a different question. Subsection (2) restricts them where the facts as the defendant believed them would still amount to another offense, and subsections (3) and (4) concern a defendant's belief that the conduct is not criminal at all, which this chapter treats separately once the mental-state analysis is complete.⁶ The first section develops route (a), beginning with the proposition that mistake is the flip side of mens rea.