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It is widely accepted that there is a level of mental derangement beyond which one is not liable for criminal acts. It is also widely accepted that a person who is insane may not be fit to answer criminal charges, even if he was of sound mind when the offence was alleged to have been committed. These two claims of insanity will have to be supported in similar ways, and judged by similar standards, but they are legally different. Therefore see Insanity defence and Insanity and unfitness to plead for further discussion of these variants.

In both cases, a defendant who claims insanity has the burden of proof, on the balance of probablilities.