For many people, especially the most religious of a fundamentalist bent, the whole idea of ethics and morality is only about rules. In this continuing exploration of our four “ethical brains” (see this earlier post for an introduction to that concept), it is practical to start with “rules and exceptions,” not because they are the earliest or dominant ethical modes, but rather because when many people in Western culture think “ethics,” they first think The Ten Commandments from the Torah/Old Testament book of Exodus.
But this was not the earliest known set of rule-based ethical commands, even in that part of the world. The ancient Babylonian Code of Hammurabi dates to about 1750 years before Christ.  The more familiar Ten Commandments that Moses presented to the Hebrew nation are usually dated from 200 to 500 years later.
The Ten Commandments do not prescribe punishments for transgressions of its rules, although there are plenty of prescriptions in the succeeding book of Leviticus and the other books comprising the Pentateuch. The Code of Hammurabi, on the other hand, presents its prescriptions specifically and vividly:
“If any one bring an accusation against a man, and the accused go to the river and leap into the river, if he sink in the river his accuser shall take possession of his house. But if the river prove that the accused is not guilty, and he escape unhurt, then he who had brought the accusation shall be put to death, while he who leaped into the river shall take possession of the house that had belonged to his accuser.” 
What is notable about the entire code, however, is not so much the rules about putting someone to death, rather the many rules that prescribe other punishments, where the king could put you to death, but spared your life because he was a “wise and gracious rule-maker.” It is the voluntary restraint by King Hammurabi that makes this document a critical turning point in human social development. Simple restraint of power is often the evolutionary precursor to more “humane” rules emerging in a society.
The classic definition of deontological ethics (literally the “study of duty”) is usually attributed to 18th-century German philosopher Immanuel Kant (1724 – 1804), although we find its seeds much earlier. Most ethical systems based on law or defined moral codes either assert or imply a duty to follow the standing rules. Deontological ethics, then, refers to any emphasis where the duty to follow the rules (called rule deontology) or the duty to act in a prescribed manner (called act deontology) outweighs any other possible alternatives. The rules can be either legal or moral in nature, but the key mindset is that faithfulness to the duty to enforce the rule, rather than any analysis of the issue, is primary.
When our brain tries to parse and interpret complex rules, it is primarily our large human “prefrontal cortex,” located behind our forehead, that kicks into high gear. When you try to go on to explain your reasoning, however, you are then more reliant on the language centers, back farther in the mid-brain, as well as the religious and cultural languages into which your memories have been encoded.
The brain is “in conversation with itself” here, reasoning out the logic in one part of the brain, and articulating the results via another part. It is no coincidence that we often need to “talk out” our reasoning to resolve complex or contradictory rules. Daniel Dennett quotes the early neurologist John Hughlings Jackson as saying, “We speak, not only to tell others what we think, but to tell ourselves what we think.” 
But ethics is more than just rules and duties, and other parts of the brain are likely telling you to disregard the rule in question. And that same prefrontal cortex, while processing the rules in question, is also simultaneously serving up potential exceptions to those rules. It turns out that, much of the time, “good people disagree” about allowable exceptions rather than rule itself. But that is another topic to come…
- Giokaris, Amalia. “The Code of Hammurabi.” Middle East and West Asia Chronology. October 13, 1999.
- “The Code of Hammurabi.” Internet Sacred Text Archive.
- Dennett, Daniel C. From Bacteria to Bach and Back: the Evolution of Minds. W.W. Norton & Company, 2017, p.345.