Defense theory is a legal concept used in criminal cases in which the accused person claims innocence by asserting that they did not commit the act in question, or if they did, they did so in self-defense. It is a form of justification in which a defendant argues that their actions were necessary to protect themselves or another person from danger. There are four main types of defense theory: self-defense, duress, necessity, and insanity. Self-defense is a claim that someone committed an illegal act, such as assault or battery, in order to protect themselves from harm. The defendant must prove that they were in imminent danger and had no other reasonable option available. Duress is the defense that a defendant committed an illegal act because they were threatened with serious harm or death by another person. Necessity is when a defendant committed an illegal act in order to prevent a greater harm from occurring. Insanity is when a defendant is claiming that they were not responsible for their actions because of a mental disorder. This is a difficult defense to prove, as the defendant must be able to demonstrate that their mental illness affected their ability to distinguish right from wrong.