The distinction between civil and criminal law is so well-established in modern American law that it seems obvious. But it was not always obvious, and there was not always a distinction.
Criminal law is, broadly speaking, law that can send violators to prison. Like many other things that we assume to be necessities of civilization, prisons are luxuries enjoyed by societies that have achieved a certain level of wealth. Although the idea of imprisonment was known in ancient societies, modern prisons as we would recognize them were not built in England until the eighteenth century and their forerunners, the workhouses, not until the sixteenth.
How do you enforce criminal law in a society without jails? There are essentially three choices: corporal punishment, status punishment, and financial punishment. Corporal punishment (punishment “by life and limb” in medieval terminology) includes execution and its more merciful (as they were considered) alternatives — mutilation, spending time in the stocks, etc. Status punishment includes banishment, outlawry, and enslavement. Financial punishment can come in various forms, is one of the easiest to administer, requires little overhead, and so was the most popular early sanction.
Before the Norman Conquest in 1066 the Anglo-Saxons used a compensation system. (I will use a simple time-line, although the timing of some of these developments is disputed.) Offenders were made to pay compensation to the victim or to his family. Payments could be either agreed upon between the parties or determined by some recognized third party such as a priest or noble. (It is not clear whether there were traditional, fixed amounts of compensation paid for particular crimes.) Payment could be enforced, where necessary, by the use of force – typically a feud between the families involved. The important point in this context is that it could all be privately arranged, without the intervention of the government.
The Normans changed things. They were able administrators but acquisitive and hard-nosed. Establishing their authority was the principal concern of the early Norman kings of England, as they tried to subdue and rule the county they had conquered. Taking to heart the traditional notion that a king had a duty to keep the peace, they decided that peacekeeping was not only the king’s obligation but also his exclusive right. Only the king, or someone acting by his authority, could proceed against and punish an offender. They were so successful in enforcing this notion that we now take it for granted that only the government enforces criminal law and punishes crime – and that private individuals who try to do so are themselves guilty of an offense against the government.
Norman cunning added a twist to this. In medieval times the king could grant – i.e., sell for money or give in return for increased feudal allegiance -- the power to enforce certain aspects of criminal law. These powers became eagerly-sought and jealously-guarded privileges. Many of the obsolete terms you read but didn’t learn about in law school refer to them; infangtheof, for example, was the right to hang a thief caught red-handed. But the king could sell not only law enforcement but also law avoidance. An accused could buy the king’s mercy — in other words, pay him money to avoid punishment. This payment was called an “amercement.” We would call it a “fine.”
Kings preferred amercements to the old, private compensation system. One reason was that amercements went to the king but compensation did not. (Kings took amercements so far that there was a specific provision about them in Magna Carta: “a freeman shall not be amerced for a slight offense, except in accordance with the degree of the offense; and for a grave offense he shall be amerced in accordance with the gravity of the offense.”) More importantly, to the Norman mind a system that allowed individuals to make private deals with offenders interfered with the king’s authority. They therefore tried to suppress the compensation system. The people resisted this. They were accustomed to compensation; they expected it; they saw it as the just, proper, and traditional way of doing things. And so the monarchs compromised: they did not give up amercements but allowed the compensation system to survive, though under the control of the courts.
And so two separate systems gradually evolved: one in which the king, or someone acting with his authority, imposed penalties on offenders; the other in which courts required offenders to compensate victims. Both were controlled by the monarch, one in aid of his own responsibilities (and financial interests), the other to accommodate traditional notions of private justice.