Gall's Law

A complex system that works is invariably found to have evolved from a simple system that worked. A complex system designed from scratch never works and cannot be patched up to make it work. You have to start over with a working simple system.

– John Gall (Systemantics: How Systems Really Work and How They Fail)

Simple rules produce complex behavior.
Complex rules produce stupid behavior.

– Andrew Hunt, 37 Signals

See also: Provoking Emergence, design patterns.