Montesquieu partly based his trias politica on the constitution of the Roman Republic, where power was divided between the consuls, the tribunes and the different assemblies of the people (the comitia and consilium plebis). However, with the fall of the Roman Republic and the rise of the empire, this separation of powers faded though the Republican constitution officially stayed in existence. The emperor was legislator, executor and judge in one. This concentration of power led to an abundance of ways of creating and interpreting the law (see Gaius, Institutes 1,5). Apart from introducing legislation in cooperation with the assemblies or the senate, the emperor could issue general laws on his own (edicta). These edicta usually dealt with matters of public law. When dealing with issues of private law, the emperor rather worked on a case-by-case basis, either responding to legal questions posed by individual citizens (rescripta) or giving judgments in specific cases as the supreme judicial instance in the empire (decreta). Both had general force of law. The Roman jurist Ulpian explains why (D. 1,4,1 pr): ‘What the emperor sees fit, has force of law. For the people commits to him and into him all of its power and authority by the Lex regia, which is passed concerning his authority.’

The judicial decisions are particularly interesting, since the emperor acted here as a judge and legislator at the same time. In his collection of imperial judgements, the Decreta, the Roman jurist Paulus provides an insight into the way decrees were used by the emperor to make and reshape the law. Sometimes he gave an interpretation of a pre-existing rule of the ius civile; an activity that can be compared to the decisions given by Dutch judges every day. But in other cases he created new law by his judgment. This frequently happened in fields of law that had not been developed in the ius civile, such as fiscal law. Since the Roman fiscus came into existence during the early years of the empire, when most of the ius civile had already been settled, the law concerning its rights and obligations had to be developed by the imperial administration. We find an example of this from the Decreta in D. 16,2,24, a decree concerning set-off (compensatio, Dutch: verrekening, 6:127 BW). In Roman law, the doctrine of set-off belonged to the general law of civil procedure. Because procedures initiated by or against the fiscus had their own law of procedure (cognitio extra ordinem), the emperor, in this case either Septimius Severus or Caracalla, was asked to decide whether set-off was also allowed in fiscal procedures. The emperor’s answer was clear: if someone could prove that the fiscus owed him money, he could set off his counterclaim against the claim of the fiscus.

Roma locuta, causa finita? Often, the imperial decisions focused on a specific question of law that had arisen in a specific case, based on a specific fact-pattern. This caused them to be too limited or too broad to serve as a general rule, and quite often the phrasing was rough around the edges. The rule that set-off was allowed against all fiscal claims was definitely too broad. The fiscus had stations all over the empire and it would inevitably give rise to long procedures and confusion if one could set off a claim from Egypt against the claim of a fiscal station in Gaul. This is why Caracalla stressed in a rescriptum that set-off was only possible with in claims against the particular station of the fiscus that had initiated the procedure (C. 4,31,1). Other sources tell us that Caracalla and his successors subsequently further restricted the types of fiscal claims against which set-off was allowed (D. 49,14,46,5).

This example shows how the Roman emperors used two of their law-making instruments (the decretum and the rescriptum) to make and reshape matters of private law on a case-by-case basis. Is this way of creating law completely unfamiliar to modern civil lawyers, shaped by Montequieu’s trias? By no means. One only has to observe the way the European Court of Justice (ECJ) has developed EU law to see the similarities. In some cases, the ECJ acts as a true supreme court, for example by delivering decreta to settle an infringement procedure brought by the European Commission against a Member State, or to judge an appeal by a company against a cartel fine. At the same time, the ECJ delivers rescripta to respond to legal questions without finally settling the dispute (preliminary reference proceedings). In developing the doctrines of EU law, it often combines both. Recently, the Dutch Supreme Court was also granted the ability to give a preliminary ruling, offering it a tool that the Roman emperors once had at their disposal. That is not to say that these contemporary courts are acting both as a judge and as a legislator, or to say that these courts employ these techniques in the same fashion or order. But it does remind us that these instruments of creating law are not unfamiliar at all to modern civil lawyers. Though the Republican constitution may survive, the Empire will strike back.