Hart endorses the formal principle of justice as desirable in any legal system. This basic principle of fairness emphasizes that laws should treat like cases alike, and different cases differently. This constancy is necessary to give moral legitimacy to a legal order. Now one should be careful not to put too much weight on this principle, as the commentator Frankena perceptively reminded us through the following example. The mad king of Transylvania had just called all of his subjects together. He showed them a huge vat of acid, which, if one were to jump into it, would cause instantaneous death. He ordered all of his subjects to jump into the vat and then jumped in himself. The formal principle of justice was adhered to; yet the principle did not successfully ground a moral system. Material principles were clearly needed as well.