"The distinction between responsible moral agents and beings with diminished or no responsibility is coherent, real, and important. It is coherent, even if in many instances it is hard to apply; it draws an empirically real line, in that we don't all fall on one side; and, most important, the distinction matters: the use we make of it plays a crucial role in the quality and meaning of our lives. [...] We want to hold ourselves and others responsible, but we recognize that our intuitions often support the judgement that a particular individual has "diminished responsibility" because of his or her infirmities, or because of particularly dire circumstances upon upbringing or at the time of action. We also find it plausible to judge that nonhuman animals, infants, and those who are severely handicapped mentally are not responsible at all. But since we are all more or less imperfect, will there be anyone left to be responsible after we have excused all those with good excuses? [...] We must set up some efficiently determinable threshold for legal competence, never for a moment supposing that there couldn't be intuitively persuasive "counterexamples" to whatever line we draw, but declaring in advance that such pleas will not be entertained. [...] The effect of such an institution [...] is to create [...] a class of legally culpable agents whose subsequent liability to punishment maintains the credibility of the sanctions of the laws. The institution, if it is to maintain itself, must provide for the fine tuning of its arbitrary thresholds as new information (or misinformation) emerges that might undercut its credibility. One can speculate that there is an optimal setting of the competence threshold (for any particular combination of social circumstances, degree of public sophistication, and so on) that maximizes the bracing effect of the law. A higher than optimal threshold would encourage a sort of malingering on the part of the defendants, which, if recognized by the populace, would diminish their respect for the law and hence diminish its deterrent effect. And a lower than optimal threshold would yield a diminishing return of deterrence and lead to the punishment of individuals who, in the eyes of society, "really couldn't help it." The public perception of the fairness of the law is a critical factor in its effectiveness."