Ius naturale est quod natura omnia animalia docuit.  nam ius istud non humani generis proprium est, sed omnium animalium, quae in caelo, quae in terra, quae in mari nascuntur.  hinc descendit maris atque feminae coniugatio, quam nos matrimonium appellamus, hinc liberorum procreatio et educatio: videmus etenim cetera quoque animalia istius iuris peritia censeri.
     Ius autem civile vel gentium ita dividitur:  omnes populi qui legibus et moribus reguntur partim suo proprio, partim communi omnium hominum iure utuntur:  nam quod quisque populus ipse sibi ius constituit, id ipsius proprium civitatis est vocaturque ius civile, quasi ius proprium ipsius civitatis:  quod vero naturalis ratio inter omnes homines constituit, id apud omnes populos peraeque custoditur vocaturque ius gentium, quasi quo iure omnes gentes utuntur.  et populus itaque Romanus partim suo proprio, partim communi omnium hominum iure utitur.  quae singula qualia sunt, suis locis proponemos.
     Sed ius quidem civile ex unaquaque civitate appellatur, veluti Atheniensium:  nam si quis velit Solonis vel Draconis leges appellare ius civile Atheniensium, non erraverit.  sic enim et ius quo populus Romanus utitur ius civile Romanorum appellamus, vel ius Quiritium, quo Quirites utuntur; Romani enim a Quirino Quirites appellantur.  sed quotiens non addimus, cuius sit civitatis, nostrum ius significamus: sicuti cum poetam dicimus nec addimus nomen, subauditur apud Graecos egregius Homerus, apud nos Vergilius.
     Ius autem gentium omni humano generi commune est.  nam usu exigente et humanis necessitatibus gentes humanae quaedam sibi constituerunt:  bella etenim orta sunt et captivitates secutae et servitutes, quae sunt iuri naturali contrariae (iure enim naturali ab initio omnes homines liberi nascebantur); ex hoc iure gentium et omnes paene contractus introducti sunt, ut emptio venditio, locatio conductio, societas, depositum, mutuum, et alii innumerabiles.
     Constat autem ius nostrum aut ex scripto aut ex non scripto, ut apud Graecos: twn nomwn oi me\n eggraphoi, oi de\ agrafoi.
     Scriptum ius est lex, plebiscita, senatusconsulta, principum placita, magistratuum edicta, responsa prudentium.  Lex est quod populus Romanus senatorio magistratu interrogante, veluti consule, constituebat. plebiscitum est, quod plebs plebeio magistratu interrogante, veluti tribuno, constituebat.  plebs autem a populo eo differt quo species a genere:  nam appellatione populi universi cives significantur, connumeratis etiam patriciis et senatoribus:  plebis autem appellatione sine patriciis et senatoribus ceteri cives significantur.  sed et plebiscita, lege Hortensia lata, non minus valere quam leges coeperunt.  Senatusconsultum est quod senatus iubet atque constituit.  nam cum auctus est populus Romanus in eum modum ut difficile sit in unum eum convocari legis sanciendae causa, aequum visum est senatum vice populi consuli.  Sed et quod principi placuit legis habet vigorem, cum lege regia, quae de imperio eius lata est, populus ei et in eum omne suum imperium et potestatem concessit.  quodcumque igitur imperator per epistulam constituit vel cognoscens decrevit vel edicto praecepit, legem esse constat:  hae sunt quae constitutiones appellantur.  plane ex his quaedam sunt personales, quae nec ad exemplum trahuntur, quoniam non hoc princeps vult:  nam quod alicui ob merita indulsit, vel si cui poenam irrogavit, vel si cui sine exemplo subvenit, personam non egreditur.  aliae autem, cum generales sunt, omnes procul dubio tenent.  Praetorum quoque edicta non modicam iuris optinent auctoritatem.  haec etiam ius honorarium solemus appellare, quod qui honorem gerunt, id est magistratus, auctoritatem huic iuri dederunt.  proponebant et aediles curules edictum de quibusdam casibus, quod edictum iuris honorarii portio est.  Responsa prudentium sunt sententiae et opiniones eorum quibus permissum erat iura condere.  nam antiquitus institutum erat ut essent qui iura publice interpretarentur, quibus a Caesare ius respondendi datum est, qui iurisconsulti appellabantur.  quorum omnium sententiae et opiniones eam auctoritatem tenebant ut iudici recedere a responso eorum non liceret, ut est constitutum.
     Ex non scripto ius venit quod usus comprobavit.  nam diuturni mores consensu utentium comprobati legem imitantur. 
     Et non ineleganter in duas species ius civile distributum videtur.  nam origo eius ab institutis duarum civitatium, Athenarum scilicet et Lacedaemonis, fluxisse videtur:  in his enim civitatibus ita agi solitum erat, ut Lacedaemonii quidem magis ea quae pro legibus observarent memoriae mandarent, Athenienses vero ea quae in legibus scripta reprehendissent custodirent.
     Sed naturalia quidem iura, quae apud omnes gentes peraeque servantur, divina quadam providentia constituta, semper firma atque immutabilia permanent:  ea vero quae ipsa sibi quaeque civitas constituit, saepe mutari solent vel tacito consensu populi vel alia postea lege lata.

Justinian. Institutes.
Book I.
Title ii. Of Natural Law, the Law of Nations, and the Civil Law

Natural law is that which nature teaches to all animals, for it is not peculiar to the human race, but is common to all animals which are produced in the air, on the earth, or in the sea. Hence comes the union of male and female, which we call marriage; hence the procreation and education of their young, for we see, in fact, that the other animals besides man act in conformity with this law as if they were acquainted with it.

§ 1. Civil law may be distinguished from the law of nations thus: all communities of men governed by laws and customs partly use their own particular law and partly that common to all men, for that law which each particular community establishes for itself, is peculiar to that community and is called the civil law, as being the peculiar law of that community; but that which natural reason establishes between all men, is equally maintained by all communities, and is called the law of nations, as being that law which is used by all nations. In this way the Romans use partly their own peculiar law, partly that common to all men. How these distinctions affect the subject we will discuss as occasion arises.

§ 2. The civil law derives its name from each particular community, as, for instance, from the Athenians,. for it would not be an erroneous expression to call the laws of Solon or Draco the civil law of the Athenians. So we call the law which is used by the Roman people the civil law of the Romans, or that of the Quirites, as being used by the Quirites; for the Romans were called Quirites from Quirinus. But whenever we do not add the name of the community to which the law belongs, we intend to be understood as referring to our own law; just as when we speak of the poet without any addition, the name under.stood amongst the Greeks would be that of the famous Homer, with us Virgil. On the other hand, the law of nations is common to the whole human race, for mankind, from the necessities of human life and the demands of the occasion, have established certain laws for themselves, for wars have arisen, followed by captivity and slavery, which are contrary to natural law, as at the outset, and by natural law all men were born free. And it is through this law of nations that almost all our contracts have been introduced, as the contract of sale, hire, partnership, deposit, loan, and innumerable others.

§ 3. Our law, like that of the Greeks, is written or unwritten. Written law consists of laws, ordinances of the plebeians, decrees of the senate, constitutions of the emperors, edicts of magistrates, and of the answers of the learned in the law.

§4. A law is that which was enacted by the Roman people on the proposition of a senatorial magistrate, such as a consul. An ordinance of the plebeians is that which was enacted by the plebeians on the proposition of a plebeian magistrate, such as a tribune. The plebeians differ from the people as the species from Its genus; for, under the term people, the whole of the citizens are comprehended, including the patricians and the senators, but the expression plebeians only includes the citizens remaining after deducting the patricians and the senators, though, after the passing of the Hortensian law, ordinances of the plebeians acquired the same force as laws.

§ 5. A decree of the senate is that which the senate orders and establishes; for when the Roman people had increased to such an extent that it was a matter of difficulty to convoke them all for the purpose of sanctioning laws, it seemed expedient that the senate should be consulted instead of the people.

§ 6. The will of the emperor has also the force of law; for by the royal law which is passed to confer authority on him, the people yield up to him all its authority and power. Whatever, therefore, the emperor directs by rescript, or decrees by a judgment, or ordains by edict, is admitted to be law; and these are what are called constitutions. Of these obviously some are personal, and are not to be drawn into precedents, since such is not the intention of the emperor; for that which he has granted to anyone as a reward for merit, or that which he has inflicted as a punishment, or that which be his given by way of assistance in an extraordinary case, does not apply beyond the particular individual. But other constitutions, are undoubtedly binding on all.

§ 7. The edicts of the praetors also carry no slight authority. These we are accustomed to include under the term honorary law, because those who bear honours, that is, the magistrates, have given rise to this description of law. The curule --diles also used to publish an edict relative to certain matters, and this edict forms a portion of the honorary law.

§ 8. The answers of the learned in the law are the decisions and opinions of those who were authorised to settle points of law, for it was formerly provided that there should be public interpreters of the law, to whom the power of expounding the law was given by the emperor, and who were called juris-consults. The unanimous decisions and opinions of these persons had such weight that it was settled by a constitution that the judge should not be at liberty to decide otherwise.

§9. The unwritten law is that which usage has approved ; for daily customs, approved by those in the habit of using them, resemble laws.