Criminal defense arises when the conditions exist to deny specific elements of the crime: the actus reus when the actions are involuntary, the mens rea when the accused is not aware of the extent of one's conduct or both. These defenses will mitigate or eliminate liability arising from a criminal offense. Insanity, automatism, and diminished responsibility are examples of such defenses. They each share characteristics but can be distinguished in their scope and application. Insanity, automatism, and diminished responsibility all play a significant role in cases where the defendant's mind is abnormal while committing a crime. The definition of abnormal will be reviewed in relation to each defense. To identify how these three defenses compare and contrast, it is first important to understand their definition and application. The appropriate defense will be used once the facts of the cases have been distinguished and have passed the legal tests. The legal test for insanity is established in the M'Naghten case: "to establish a defense... of insanity it must be clearly demonstrated that, at the time he committed the act, the accused party was suffering from such a defect of reason , from disease of the mind, such as not knowing the nature and quality of the act he was doing, or, if he knew it, not knowing that he was doing what was wrong”. Specifically, the defect of reason occurs when the defendant is incapable of exercising normal reasoning. The defect of reason requires the instability of reasoning rather than the failure to exercise it at the time when the exercise of reason is possible. The defendant was clinically depressed and, in a moment of distraction, stole items from a supermarket... in the middle of paper... the prosecution must disprove automatism beyond reasonable doubt. It is clear that insanity, automatism and diminished responsibility share similarities and differences in scope and definition. Insanity and automatism are very similar in that both are complete defenses (with different outcomes) that exist when a defendant lacks the necessary actus reus or mens rea, while diminished responsibility is a partial defense that applies only to murder . The source of the defendant's mental abnormality is the main point of distinction among all defenses. Whether the abnormality is internal, external, or a diagnosed medical condition will play a significant role in where the defense can be used. As defenses they are all used for a similar reason, namely to eliminate or reduce liability for criminal offences.