Funny thing. With first year Ancient History courses in University, those that the jocks and the screwheads (engineers) take to get their humanities requirement, the standard issue lesson is that Rome fell by degrees through the 4th and 5th centuries, and was replaced with the Dark Ages. End of story. There's more to it than that, but with sixty-eighty people per class, the emphasis is just to teach people that there was a Rome, that it was an important part of our past and that yes, people actually breathed and had thoughts that are still relevant. That's enough for them. But for the gentle reader, I'll try and do better.
When I was a teenager, and still reading through my parent's library, I found there a terrific book - written for victims of the English school system - called 1066 And All That. It makes great hay of what English history an adult will be expected to remember from learning English history, and what they will be expected to remember wrongly. I was not a product of that system, being Canadian, but I did know the history and I did chuckle my way through the book. I'm sure I have my copy around somewhere.
A similar book on Roman history could easily be entitled, And Then Came 476, Full Stop. Because I remember being taught somewhere before seriously becoming a classics scholar that the Roman Empire ended in 476 A.D. It is the date of Rome's 'destruction' by Odoacer the Goth ... but Rome had been 'destroyed' in 410, just 66 years earlier. The difference was that Odoacer removed the last Western emperor of Rome, so that the title, not the political entity, came to an end.
There was another emperor of Rome at the time, who resided in Constantinople, who went on living and being replaced with other emperors for nearly 1000 years after 476. They went on calling themselves the Roman Empire, although we call them the Byzantines. The name is not what's important, however. What is important is that individuals within that society continued to live according to Roman Law. As they did everywhere else in Europe, dead emperor or no.
Note that I do not say, 'Christian Law.' Christian law is not, and never has been, of any relevance to western society. Our society does not follow the precepts of Deuteronomy, Leviticus or any of the Old Testament except by coincidence - and damn few coincidences at that (feel free to read and compare). No, western laws are based upon the same principles of justice established in and around the sixth century BCE, with the fall of the Kings of Rome, that preceded the Republic of Rome, that preceded the Empire.
The genius of Roman Law was that previously unthought-of circumstances, which had never come up before, could be decided by a judge ... and then that decision would itself become law. Strange circumstances had previously been brought before the king, who made a judgment on the matter ... but the next king in line was not in any way compelled to view the same matter the same way. Thus, the law changed from monarch to monarch.
But a court of law was not limited to the bloodline of a reigning king. Once the judgment was made, it set a precedent - and all future judges were bound to accept the precedent as law. As such, the social limitations on individual behavior improved with time, not according to a individual's whims, but according to a systematic, and far less passionate, interpretation of daily events.
Other democracies were playing with the same structure - the Greeks of course, and certainly the Chinese, whose establishment of a hierarchical bureaucracy bound the state with so many laws that social change was restrained and suppressed. But I don't want to talk about the Chinese today - perhaps another time.
And about the Greeks - it was Aristotle that said "The law is reason free from passion." But the Greeks were such hopeless, squabbling children during the entire time of the Rome Republic's steady ascendance, it's hard to argue that the rest of Greek society understood the least thing about being free from passion. Their democracies did not tend to be widespread or consistent, repeatedly collapsing into oligarchies and worse ... so that while Greek thought had great influence upon Roman Law, the idea itself retained a distinctive Roman flavour.
When the Republic collapsed under the will of dictators (Caesars, then emperors), the law carried on. In the first century A.D. the law became a tool for ambitious generals to declare themselves emperors ... and increasingly the emperors became less and less bound by it. People point to Caligula as an evil emperor, but he reigned for only four years and was hardly a patch on others who would come later, such as Domitian, Commodus or Elagabalus. (Caligula is only hot shit because we have multiple sources for him, whereas scholarship had declined as the emperors got worse).
But whatever the emperors did or did not do within the confines of the law was a matter for that small group within the emperor's reach. Those vast, backward regions of the empire were hardly affected by how many self-made gods sat upon the throne. Daily, ordinary disputes like boundary lines, contracts and assault cases were carried along in the same way that they had been during the Republic. Precedents continued to be steadily added to the system, though with the demise of central authority the listings for these precendents failed to become generalized throughout the empire. Eventually, law became a matter of isolated pockets doing whatever seemed correct for them. The technological institution (for I continue to point out these breakthroughs are a techology, as used in Civ IV) remained unchanged.
And when the western emperor ceased to be, and Gaul, Iberia and North Africa fell under the sway of the Franks, the Visigoths and the Vandals, there weren't enough foreign tribesmen to fundamentally change how such matters were decided. In fact, western Europe swallowed the tribes up, turning them Christian and making them into 'Roman' kings, teaching them how to enact their authority according to the dictates as established by judges who had lived and set precedents centuries before.
The 'Byzantine Empire,' the Eastern Empire of Rome, retained a great deal of central authority over its dispossed regions. One particular emperor, Justinian I (6th century), saw the difficulties arising from each region having hundreds of precedents shared by no other region, and so demanded that the laws be reunited under a singular code. Justinian's Code, the Body of Civil Law, would retain vast sections of the law that would be lost to the west ... who had no reason or purpose to retain laws applying to urban planning and urban life, since 'urban' was fast becoming a non-existent thing. Except for the creation of the Code, the more complex parts of Roman Law, devised under the Republic and during the Empire, would have been lost completely.
It would be seven centuries after Justinian before this code would be widely introduced back into western society. The growing city states of Italy in the 1200s, stymied for a reasonable methodology to deal with hundreds of matters arising from urbanization, embraced wholesale the written laws deseminating out of the East, using them to re-establish Roman Laws back into their lives. Changed somewhat, with an additional seven centuries of precedents, the law would continue to change up until the present day.
For yes, we too live under Roman Law, as I alluded to earlier. Judges, juries, the pomp and ceremony, each participant's role and limitations - all put in place millenia ago. And every depiction that occurs in every fictional setting, from the Cardassians' guilty-before-innocent supposition to the banal strains of Judging Amy, follows lockstep the institutions of judge, defendant, prosecutor, witness and so on.
When considering your D&D setting, remember that you are the product of thousands of years of thinking, inducing you to believe that any civilized, decent society should possess these processes to determine guilt or innocence. But how did they come about when your world had no Rome? This world's only alternative method to Roman Law has been the Chinese - whom I did not talk about, remember - which fell apart in 1840 when the bureaucratic system collapsed to European gunpowder, and disappeared entirely in 1908. China today is a product of western thinking.
It isn't fair, I'm realizing, that I haven't described the Old Bureaucracy. However, this post has been going on long enough (what do you expect if you're going to discuss law?) ... and I want to wrap it up with this:
If the gentle reader is not going to have a system of law based on any justice we understand (and let's face it, we've just got the one), the only alternative is instituting law-based-on-a-whim. And that is a hard system to operate in as a sandbox player. We're used to being able to predict what is, and what is not, illegal - and a lot of laws being sprung on a player out of the blue can really hurt a campaign. I've run into problems like that just speaking of laws that were actually in existence in the 17th century, that are gone now due to increased freedom with democracy.
On a lot of levels, it is easier for the party, and easier for the DM, to retain a loose presentation of the law as we understand it now; which is, I've found, what DMs tend to do. Don't steal, don't kill, don't cast magic at others ... those things are obvious to anyone. But change the laws - and indeed, invent your own, non-Roman system - and watch the confusion and unhappiness grow.