Roman jurisprudence was finally incorporated in Justinian’s _Digest_, which opens with a statement of the most general principles, even those which would have hung in the air but for the Roman genius of logical and practical application to the concrete instance. “Jus est ars boni et aequi”--it is better to leave these words untranslated, such is the wealth of significance and connotation which they have acquired. “Justitia est constans et perpetua voluntas jus suum cuique tribuendi. Juris praecepta sunt haec: honeste vivere, alterum non laedere, suum cuique tribuere. Jurisprudentia est divinarum atque humanarum rerum notitia, justi atque injusti scientia.” The first pregnant phrase is from the older jurist Celsus; the longer passage is by the later Ulpian, and may be taken as an expansion of the first. Both the one and the other expressed the most advanced and philosophic ethics of the ancient world. They are both in the first chapter of the _Digest_, wherein they become enactments. An extract from Paulus follows: “_Jus_ has different meanings; that which is always _aequum ac bonum_ is called _jus_, to wit, the _jus naturale_: _jus_ also means the _jus civile_, that which is expedient (_utile_) for all or most in any state. And in our state we have also the praetorian _jus_.” This passage indicates the course of the development of the Roman law: the fundamental and ceaselessly growing core of specifically Roman law, the _jus civile_; its continual equitable application and enlargement, which was the praetor’s contribution; and the constant application of the _aequum ac bonum_, observed perhaps in legal rules common to many peoples, but more surely existing in the high reasoning of jurists instructed in the best ethics and philosophy of the ancient world, and learned and practised in the law. Now notice some of the still general, but distinctly legal, rather than ethical, rules collected in the _Digest_: The laws cannot provide specifically for every case that may arise; but when their intent is plain, he who is adjudicating a cause should proceed _ad similia_, and thus declare the law in the case.[348] Here is stated the general and important formative principle, that new cases should be decided consistently and _eleganter_, which means logically and in accordance with established rules. Yet legal solecisms will exist, perhaps in a statute or in some rule of law evoked by a special exigency. Their application is not to be extended. For them the rule is: “What has been accepted _contra rationem juris_, is not to be drawn out (_producendum_) to its consequences,”[349] or again: “What was introduced not by principle, but at first through error, does not obtain in like cases.”[350]