Rules-Based Systems
(the baseline)
A rules-based system runs on rules a human wrote. If a thing is true, do this. If not, do that. Every banking app, every tax calculator, every airline booking screen you have ever used runs like this. Rules-based systems are most of the software on Earth.
An email arrives. Each rule checks one thing. The email lands in Spam or Inbox.
The process
A programmer writes the rules ahead of time. A rule has two parts. The test, and the action. The test looks at one thing, like “does the email contain the word ‘prize’?” The action is what happens if the test is true.
Each email passes through the list of rules one by one. The first rule that matches decides the email’s fate. If no rule matches, the email goes to the Inbox by default.
Rules-based systems are fast, cheap, and clear. You can read every rule. You can edit any rule. You always know why the computer did what it did.
A familiar example
Think of a restaurant menu with allergy notes. “Contains nuts. Contains dairy. Vegetarian.” A waiter scans the notes against a guest’s allergies. If a note matches, the waiter flags the dish. That is a rules-based system. A human wrote the rules. The waiter runs them.
Variants include
Decision trees (the hand-written kind)
A decision tree is a rules-based system drawn as a flowchart. Each branch is a rule. You walk from the top of the tree to the bottom, answering yes or no at each fork. Tax software uses these. Medical triage uses these.
Expert systems
Expert systems are rules-based systems with hundreds or thousands of rules, written by people who are experts in a field. Doctors used to build them to help diagnose patients. They worked on narrow problems. They failed on anything the experts had not thought of.
The breaking point
Rules-based systems break the moment the world stops matching the rules. A new kind of spam email appears. The rules miss it. The spam lands in your inbox. Someone has to notice, write a new rule, and deploy the update. Every time. For every new kind of spam. This is why email filters used to be terrible, and why people started looking for something better.
Your takeaway
Most of the software in your life is still rules-based. It is reliable, boring, and understood. It is the baseline that makes everything else impressive by comparison.