The basic moral issue regarding the insanity defense is whether it is just to hold responsible and punish a person who was extremely crazy at the time of the offense.4 Those who believe that the insanity defense should be abolished must claim either that no defendant is extremely crazy at the time of the offense or that it is morally proper to convict and punish such people. Neither claim is easy to justify.

In all societies some people at some times behave crazily—that is, the behavior at those times is recognizably, aberrantly irrational. A small number of these people behave extremely crazily on occasion, including those times when an offense is committed. A hypothetical defendant with a delusional belief that he is the object of a murderous plot, who kills one of the alleged plotters after hallucinating that he hears the plotter's foul threats, is crazy. Such cases are rare, but clearly exist; the influence of extreme craziness on some criminal behavior cannot be denied.

 For hundreds of years the common law has recognized the unfairness of holding some crazy persons responsible for their criminal behavior.5 The legal test for insanity, designed to identify the appropriate persons to be excused, has changed over the years. Whether the test seeks to excuse only those akin to wild beasts or also those who lack substantial capacity to conform their conduct to the requirements of law, the moral perception has remained constant: at least some crazy persons should be excused. Those who would abolish the defense must argue that no sound principles underlie the law's consistent retention of the defense. That most past discussions of the issue have failed clearly to identify such principles6 is hardly an argument that they do not exist. I maintain that such sound principles do exist; some persons whose craziness influences their criminal behavior cannot fairly be held responsible and thus do not deserve punishment.

To justify the moral necessity of the insanity defense, I must set forth some assumptions I make about our system of criminal justice. Conviction and punishment are justified only if the defendant deserves them. The basic precondition for desert in all contexts, legal and otherwise, is the actor's responsibility as a moral agent. Any condition or circumstance that sufficiently compromises responsibility must therefore negate desert; a just criminal law will incorporate such conditions and circumstances in its doctrines of excuse. A coherent, purely consequentialist theory of criminal justice, while conceivable, is so unattractive morally that few persons, including most critics of the insanity defense, adhere to such a position.7 Moreover, our present system clearly rests on a much different basis:8 our system of criminal justice accepts desert, whether viewed as a defining or limiting principle,9 as fundamental to guilt and punishment.

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The insanity defense is rooted in moral principles of excuse that are accepted in both ordinary human interaction and criminal law. Our intuition is that minimal rationality (a cognitive capacity) and minimal self-control or lack of compulsion (a volitional capacity) are the essential preconditions for responsibility.10 Young children are not considered responsible for the harms they cause precisely because they lack these capacities.11 Similarly, adults who cause harm while terrifically distraught because of a personal tragedy, for instance, will typically be thought less responsible and culpable for the harm than if they had been normally rational and in control.12 Aristotle recognized these fundamental requirements for responsibility by noting that persons may be less blameworthy for actions committed under the influence of mistake (a cognitive problem) or compulsion (a so-called volitional problem).13

Criminal law defenses that focus on the moral attributes of the defendant are based on these same intuitions and principles. Even if the defendant's conduct fulfills the usual requirements for prima facie guilt—that is, act, mental state, causation, result—the defendant will be found not guilty, not culpable, if the acts committed were the products of cognitive (e.g., infancy) or volitional (e.g., duress) circumstances that were not under the defendant's control. These defenses are considered relevant at the time of guilt determination as well as at the time of sentencing. It would be indeed illogical in a criminal justice system based partly on desert to hold that a defendant with a valid claim of duress is culpable (because he or she intended to do the compelled act), but then to decide to release the defendant because he or she does not deserve punishment. To convict a person with a meritorious defense would offend our conception of the relationship between legal guilt and blameworthiness. A person acting under duress is not culpable, although it is unfortunate that a prohibited act has been committed.

In sum, the moral basis of the insanity defense is that there is no just punishment without desert and no desert without responsibility. Responsibility is, in turn, based on minimal cognitive and volitional competence. Thus, an actor who lacks such competence is not responsible, does not deserve punishment, and cannot justly be punished.