A historical overview demonstrates how the insanity defense has remained a highly controversial issue in Anglo-American substantive criminal law. The author explores the theoretical foundation of the insanity defense in terms of the meaning conveyed by conduct. The insanity defense arises when a defendant, due to mental illness, lacks the cognitive or volitional capacity to guide his conduct according to moral and legal factors so as to avoid harming protected interests of the individual or the community. It is this lack of capacity that prompts us, through a jury, to not interpret in the defendant's conduct a disrespect for those interests. The mental elements of a crime are the law's mechanisms for interpreting the significance of disrespect expressed by the conduct of normal offenders. Taken together, mental elements and excuses, including the insanity defense, operate as the law's intricate system for interpreting the quality of respect or disrespect expressed by the defendant's conduct. This perspective helps explain why the insanity defense has endured: the quality of respect or disrespect for legal protected interests expressed by a defendant's conduct lies at the heart of criminal responsibility, and the insanity defense helps the jury assess such respect or disrespect. 323 footnotes. (Author summary modified)