KISS
Acronymkeep-it-simple, keep it simple stupidDefinition
"Keep it simple, stupid" — the design principle that most systems work best when kept simple, so simplicity is a goal to design for and complexity something to justify, not the other way around
Kelly Johnson's rule at Lockheed's Skunk Works: a jet should be repairable by an average mechanic with ordinary tools in the field. In software the field mechanic is the next reader — a colleague in six months, or the author, or an agent — and the test is whether they can fix it with what they have. Simple is not the same as easy: the simple design is often the harder one to arrive at, which is what the refactor step of TDD is for.
Complexity is a cost paid by every reader, forever, and the person who added it is the one least able to see it. Ask what the simplest thing that could work is, build that, and let the case for more make itself.