The fully competent, culpable, and malevolent Spike attacks Mother Beneficence with a tire iron in order to steal the alms she has collected for the poor. Mother Beneficence defends herself by kicking Spike in the shins, and Dudley Doright rushes to her rescue, smiting Spike sharply, knocking him to the ground, and holding him until the police arrive. Dudley acts solely for the purposes of preventing harm to Mother Beneficence and bringing Spike to Justice.

Both Mother Beneficence and Dudley fulfill the offense elements for assault in that they purposely cause bodily injury to another human being. Most readers, jurors, and theorists would probably agree, however, that their actions are justified. The Model Penal Code (MPC) provides justification defenses of self-defense for Mother Beneficence and defense-of-others for Dudley. These defenses would not only exempt Mother Beneficence and Dudley from punishment, but also ratify their defensive force as acceptable under the circumstances.

Most theorists accept the general characterization of justification defenses as affirmative criminal defenses that ratify the defendant's conduct as acceptable under the circumstances, despite fulfilling all offense elements for some criminal offense. This formulation distinguishes justification defenses from excuses that exempt the defendant from punishment due to some disability, but do not mark the defendant's conduct as acceptable under the circumstances.

Despite the broad acceptance of this basic distinction, important questions remain regarding both the general class of justification defenses and specific defenses that fall within this category.