The recognition of these simple modes of contracting obligations, and perhaps the knowledge that certain rules of law obtained among many peoples, fostered the conception of common or natural justice, which human reason was supposed to inculcate everywhere. Such a conception could not fail to spring up in the minds of Roman jurists who were educated in Stoical philosophy, the ethics of which had much to say of a common human nature. Indeed the idea _naturalis ratio_ was in the air, and the thought of common elements of law and justice which _naturalis ratio inter omnes homines constituit_, lay so close at hand that it were perhaps a mistake to try to trace it to any single source. Practically the _jus gentium_ became identical with _jus naturale_, which Ulpian imagined as taught by nature to all animals; the _jus gentium_, however, belonged to men alone.[344] Thus rules which were conceived as those of the _jus gentium_ came to represent the principles of rational law, and impressed themselves upon the development of the _jus civile_. They informed the whole growth and application of Roman law with a breadth of legal reason. And conceptions of a _jus naturale_ and a _jus gentium_ became cognate legal fictions, by the aid of which praetor and jurisconsult might justify the validity of informal modes of contract. In their application, judge and jurist learned how and when to disregard the formal requirements of the older and stricter Roman law, and found a way to the recognition of what was just and convenient. These fictions agreed with the supposed nature and demands of _aequitas_, which is the principle of progressive and discriminating legal justice. Law itself (_jus_) was identical with _aequitas_ conceived (after Celsus’s famous phrase) as the _ars boni et aequi_. The Roman law proper, the _jus civile_, had multifarious sources. First the _leges_, enacted by the people; then the _plebiscita_, sanctioned by the Plebs; the _senatus consulta_, passed by the Senate; the _constitutiones_ and _rescripta[345] principum_, ordained by the Emperor. Excepting the _rescripta_, these (to cover them with a modern expression) were statutory. They were laws announced at a specific time to meet some definite exigency. Under the Empire, the _constitutiones principum_ became the most important, and then practically the only kind of legal enactment. Two or three other sources of Roman law remain for mention: first, the _edicta_ of those judicial magistrates, especially the praetors, who had the authority to issue them. In his edict the praetor announced what he held to be the law and how he would apply it. The edict of each successive praetor was a renewal and expansion or modification of that of his predecessor. Papinian calls this source of law the “_jus praetorium_, which the praetors have introduced to aid, supplement, or correct the _jus civile_ for the sake of public utility.”