This defense is rare, generally reserved for serious cases, or when no other valid defense exists. (last resort)

The defense you're referring to sounds like the "insanity defense." This legal defense is indeed rare and is used in criminal cases when a defendant claims that they should not be held responsible for their actions due to psychiatric illness or mental incapacity at the time of the alleged criminal act. To use the insanity defense, the defense must typically show that the defendant was either unable to distinguish right from wrong or was unable to control their actions due to their mental state. It is a complex defense often seen as a last resort, reserved for serious cases where no other valid defense can be found or applied.

Extra: The insanity defense has a long history and is rooted in the idea that culpability for a crime requires a certain mental state, or mens rea. Different jurisdictions have different standards for what qualifies as legal insanity. In the United States, for example, two of the most famous standards include the M'Naghten Rule, which focuses on the defendant's ability to understand the nature and quality of the act and to know it was wrong, and the Model Penal Code test, which includes the inability to appreciate the criminality of an action and the inability to conform one's conduct to the law.

The use of the insanity defense is often controversial because it raises questions about free will, responsibility, the nature of mental illness, and the ability of psychiatric expertise to reliably assess someone's mental state at the time of a crime. When a defendant is found not guilty by reason of insanity, they are typically committed to a mental health institution for treatment rather than being sent to prison. The length of their stay in the institution may depend on their progress and the determination of their risk to themselves and others.