Ignorance or Mistake

Fundamentals of Criminal Law by Adam J. McKee

In the realm of law, individuals often put forth defenses based on their understanding or misunderstanding of certain situations. Mistakes can be broadly categorized into two types: mistakes of fact and mistakes of law. Each type has distinct implications and consequences when presented in court. Understanding the difference between the two is crucial when considering defenses in a legal context, as they can significantly influence the outcome of a case.

Mistake of Fact

A mistake of fact occurs when you believe certain circumstances to be true, but they aren’t. For instance, if you take someone else’s umbrella thinking it’s yours, you’ve made a mistake of fact. This kind of mistake can serve as a defense in two main situations:

  1. Statutory Provisions: If the law, or statute, specifically mentions that a mistake of fact can be used as a defense for a certain crime.
  2. Justifiable Belief: If your incorrect belief about a fact can justify your actions, it might serve as a defense.

However, it’s essential to note that a mistake of fact might not always work as a defense. For crimes where the intention doesn’t matter, like certain traffic offenses, claiming you were mistaken might not help. Imagine telling a judge you thought the speed limit was 70 mph when it was 50 mph; it’s unlikely to work in your favor.

Mistake of Law

Mistakes about the law are a bit more complicated. This kind of mistake happens when you misunderstand the laws or rules governing a situation. Here’s the catch: just not knowing the law, in most cases, won’t serve as a defense. But there are exceptions:

  1. Official but Incorrect Law Sources: If you were misled because of an official source like court decisions or governmental interpretations that were wrong, then you might have a defense.
  2. Specific Statutory Interpretation: If your mistake was about how a particular law should be interpreted, it might be considered in your defense, especially if it disproves any wrongful intent on your part.

However, it’s essential to be cautious. For instance, if you get the wrong advice from a lawyer and act on it, that usually won’t work as a “mistake of law” defense.

In general, mistakes of fact tend to be more accepted as defenses than mistakes of law. Knowing the difference and when each might be applicable is essential for anyone navigating the legal system.

MPC’s Treatment of Mistake Defenses

The Model Penal Code (MPC) provides a comprehensive framework for understanding and applying defenses based on mistakes. This legal guide, which serves as a benchmark for many jurisdictions, delineates the nuances between mistakes of fact and mistakes of law, clarifying the circumstances under which each can be invoked as a defense.

Mistake of Fact under the MPC

According to the MPC § 2.04(1), a mistake of fact can be a defense if it negates the required mental state necessary for commission of the offense. This means if an individual commits a crime under a mistaken belief of a certain fact and that mistaken belief, if true, would make the act non-criminal, then such a mistake of fact could provide a defense. It’s important to note, however, that the defense is not available if the defendant would still be culpable of another offense had the circumstances been as he or she supposed. For example, if someone mistakenly believed they were taking their own bag, when in fact they were taking someone else’s, they could potentially use mistake of fact as a defense against a theft charge, given they lacked the intent to steal.

Mistake of Law under the MPC

Mistake of law, on the other hand, is generally not a defense to criminal liability. The age-old maxim “ignorance of the law is no excuse” typically holds true. However, the MPC does carve out certain exceptions. According to MPC § 2.04(3), a mistake of law will be a valid defense if the mistaken belief was due to a reliance on an official statement of the law, such as a statute, judicial decision, or an official interpretation from a public officer or body. For instance, if someone acted based on an outdated law they found in an official government publication, they might be able to use mistake of law as a defense if charged.

Limitations and Considerations

However, the MPC is clear about the limitations. A defendant cannot use a mistake of law defense based on their own misinterpretation of the law or based on incorrect legal advice they received. Section 2.04(2) emphasizes that a mistake of law defense is not available if the defendant is aware of the provision in question or if they should have been aware after making a reasonable attempt to discover it. The MPC, in essence, seeks to strike a balance, ensuring that genuinely misled individuals have a potential defense, but also ensuring that the defense isn’t misused or invoked frivolously. This emphasizes the importance of due diligence in understanding and interpreting the law, especially when taking actions that have significant legal ramifications.


In legal proceedings, defenses often arise from genuine misunderstandings or mistakes. These mistakes are either about certain facts (mistakes of fact) or misunderstandings of the law itself (mistakes of law). The difference between them is vital as they impact the outcomes of legal cases differently. Mistakes of fact relate to misjudging a situation, like thinking you’re taking your umbrella when it’s someone else’s. Meanwhile, mistakes of law occur when there’s a misinterpretation of legal rules. However, simply being unaware of the law usually isn’t a valid defense.

The Model Penal Code (MPC) offers detailed guidelines on these defenses. According to MPC, a mistake of fact can be a defense if it negates criminal intent, but there are limitations. For mistakes of law, the defense is typically not valid unless there’s reliance on an official but mistaken source of law. The MPC emphasizes due diligence in understanding the law and warns against misusing the defense based on mistakes.

Modification History

File Created:  07/17/2018

Last Modified:  09/26/2023

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This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.

Open Education Resource--Quality Master Source License


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