Can “I Didn’t Know” Be a Defence? Understanding Ignorance of Law and Mistake of Fact

Can a person avoid criminal responsibility by saying, “I did not know it was against the law”?

Generally, the answer is no. The longstanding legal principle ignorantia juris non excusat: ignorance of the law is no excuse, means that a person cannot ordinarily escape criminal responsibility simply because they did not know that their conduct was prohibited.

The position is different, however, when a person acts under a genuine mistake about an important fact. In some circumstances, a mistake of fact may prevent the prosecution from establishing the intention, knowledge or other mental element required for an offence.

Understanding the distinction between ignorance of law and mistake of fact is therefore essential. Although both may involve a person saying, “I did not know,” the law does not necessarily treat them in the same way.

What Is Ignorance of Law?

Ignorance of law occurs when a person understands what they are doing but does not know that the conduct is unlawful.

For example, a person may deliberately perform an act but later claim that they were unaware of the law prohibiting it. That lack of legal knowledge will not ordinarily provide a defence.

The principle exists for practical reasons. If a person could avoid responsibility merely by claiming not to know the law, enforcement would become extremely difficult. Courts would regularly be required to determine what each person knew about a particular legal rule before deciding whether the person could be held responsible.

The principle also discourages deliberate avoidance. A person should not be able to escape liability by refusing to learn about legal obligations connected to their activities.

However, saying that ignorance of law is generally not a defence does not mean that every allegation automatically results in criminal responsibility. The prosecution must still prove the elements of the particular offence to the standard required by law. The conduct must also have been prohibited by a valid written law at the time it occurred.

Section 36(12) of the Constitution of the Federal Republic of Nigeria provides that a person cannot be convicted of a criminal offence unless the offence is defined and its penalty prescribed in a written law. The rule against ignorance of law therefore does not permit punishment under an unpublished, undefined or nonexistent criminal offence.

What Is a Mistake of Fact?

A mistake of fact occurs when a person misunderstands or is unaware of an important factual circumstance surrounding their conduct.

Consider two identical black bags placed beside each other at an event. If a person picks up another attendee’s bag honestly believing it is their own, that mistake may be relevant to whether the person intended to steal.

The person knows that taking another person’s property is prohibited. The mistake is not about the law; it is about the ownership of the particular bag.

Section 25 of Nigeria’s Criminal Code Act addresses this distinction. It provides that a person who acts or fails to act under an honest and reasonable—but mistaken—belief in a state of affairs is not criminally responsible to any greater extent than if the circumstances had been as the person believed them to be. The section also recognises that the operation of this rule may be excluded by the law governing a particular offence. 

A mistake of fact is therefore not a universal or automatic defence. Its effect depends on:

• whether the mistake was genuinely held;
• whether the belief was reasonable in the circumstances;
• whether the mistaken fact relates to an element of the alleged offence;
• what the applicable law requires the prosecution to prove; and
• whether the law creating the offence limits or excludes reliance on the mistake.

The exact legal position may also depend on the criminal law applicable in the relevant part of Nigeria and the particular offence charged. Anyone facing an investigation or prosecution should obtain advice from a qualified legal practitioner rather than rely on a general explanation.

The Mistake Must Be Relevant

Not every factual mistake affects criminal responsibility. The mistake must relate to something legally significant.

Suppose a person deliberately takes a phone belonging to someone else but mistakenly believes that the phone is a more expensive model. The mistake concerns the phone’s value or model, not its ownership. It would not ordinarily remove the intention to take another person’s property.

By contrast, if the person honestly and reasonably believes that the phone belongs to them, that mistake goes directly to the question of whether they intended to take property belonging to another person.

The central question is not simply whether a mistake occurred. It is whether, if the facts had been as the person believed, their conduct would have been innocent or less blameworthy under the applicable law.

Honest and Reasonable Belief

A person cannot avoid responsibility merely by asserting, “I made a mistake.” Courts consider the evidence and circumstances surrounding the claim.

An honest belief is one the person genuinely held at the relevant time. A belief invented after an arrest or accusation will not qualify simply because it has been described as a mistake.

Reasonableness concerns whether the belief was one that could sensibly have been held in the circumstances. Courts may consider what the person saw, heard or was told; the steps they took to verify the situation; and whether they ignored obvious warning signs.

For example, picking up an identical bag from the place where a person had left their own may support a claim of honest and reasonable mistake. Taking a clearly labelled bag from a locked room may be much harder to explain in the same way.

Mistake of fact should therefore not be confused with carelessness, recklessness or deliberate blindness. A person who suspects the truth but intentionally avoids confirming it may have difficulty establishing that the belief was honest and reasonable.

Mistake of Fact and Criminal Intention

Many criminal offences require proof of both prohibited conduct and a particular state of mind. This state of mind may involve intention, knowledge, recklessness or another mental element specified by law.

A mistake of fact can matter when it prevents the required state of mind from being established. If a person genuinely and reasonably misunderstands a key circumstance, the prosecution may be unable to prove that the person possessed the necessary intention or knowledge.

This does not mean that a mistaken person can never be held responsible. The person may still be liable for another offence, particularly where the law punishes negligence or imposes responsibility without requiring proof of a particular intention.

Each case must therefore be considered under the wording of the relevant offence. General examples are useful for explanation, but they cannot replace an examination of the evidence and applicable law.

Why Legal Awareness Still Matters

Although ignorance of law is generally not a defence, access to understandable legal information remains essential to a fair society.

People are more likely to comply with the law when legal rules are clear, public and accessible. Legal education helps individuals understand their responsibilities, recognise unlawful conduct and make informed decisions.

Legal awareness is equally important when people come into contact with law-enforcement and justice institutions. A suspect may not understand the reason for an arrest, the right to remain silent, the right to consult a legal practitioner, the conditions governing bail or the procedures for challenging unlawful detention.

The Administration of Criminal Justice Act 2015 contains safeguards intended to protect suspects and promote fair criminal proceedings. Among other requirements, it addresses informing suspects of their rights, access to legal representation, the recording of statements and humane treatment in custody. 

Rights are far less effective when the people entitled to exercise them do not know that they exist. This is why public legal education, legal aid and independent monitoring of justice institutions remain indispensable.

The Law Applies to Public Authorities Too

The principle that citizens are expected to comply with the law must not operate in only one direction. Police officers, correctional authorities, prosecutors and other public officials must also act within the powers granted to them by law.

A person’s lack of legal knowledge should never be exploited to justify intimidation, unlawful detention, denial of legal representation or other abuses of authority.

Public officials must respect constitutional protections and follow the procedures established for arrest, investigation, detention, bail and prosecution. Effective accountability is necessary when those powers are misused.

The rule of law requires both citizens and institutions to be subject to legal limits. It cannot mean strict enforcement against the public while unlawful conduct by officials is ignored.

AWJAI’s Role in Promoting Access to Justice

Through legal interventions, monitoring of police and detention facilities, correctional-centre interventions and public legal education, AWJAI works to help vulnerable people understand and navigate the justice system.

This work recognises that access to justice means more than the existence of courts or legal rules. It requires people to understand their rights, obtain appropriate legal assistance and seek remedies when those rights are violated.

Public legal education does not remove individual responsibility. Instead, it helps people understand both their duties under the law and the protections available to them.

Conclusion

“I did not know the law prohibited it” will not ordinarily excuse criminal conduct. A genuine mistake about an important fact, however, may affect criminal responsibility where it prevents the prosecution from proving an essential element of the offence.

The distinction is important:

• ignorance of law concerns not knowing that conduct is legally prohibited; and
• mistake of fact concerns misunderstanding a factual circumstance connected to the conduct.

Whether a mistake of fact succeeds as a defence depends on the applicable law, the nature of the alleged offence and the evidence in the particular case. It cannot be determined by the claim of mistake alone.

A fair justice system must do more than enforce legal rules. It must ensure that those rules are publicly available, clearly communicated and applied equally. It must also ensure that people accused of offences can understand the allegations against them, obtain legal assistance and exercise their constitutional rights.

Access to justice means having rights that exist not only on paper, but also in practice. The law must be enforceable, but it must also be understandable, accessible and binding on everyone—including the institutions entrusted with enforcing it.

By Thomas Oreoluwa Esther