Many types of legal defense are used by lawyers who represent the defendants in court trials. When the defense presents arguments or statements of fact that support his or her case or in any way diminish the case presented by the prosecution, this is referred to as an affirmative defense. It is called affirmative defense since it primarily does not just negate whatever argument is presented by the prosecution, but presents a substantive, original argument that serves to support the claims made by the defendant or to counter and diminish any claims made by the prosecution.
One form of affirmative defense is the argument of self-defense. A defendant may be acquitted of the charges brought up against him or her if it can be proven that he or she had honestly and reasonably believed that he or she had acted to protect himself or herself from any harm that could have been inflicted upon him or her. Self-defense is a simple, affirmative defense that does not primarily contradict the arguments of the prosecution, but diminish claims of assault, murder, or manslaughter.
Another form of affirmative defense is mistake of fact wherein the defendant acted based on his or her observations that he or she believed warranted such an act despite the actual fact that it may not have been warranted. A more controversial form of affirmative defense involves the use of the insanity defense, which involves a defendant claiming temporary insanity at the moment he or she committed the crime.