Borrowed from the excellent book, The Personal MBA
A complex system that works is invariably found to have evolved from a simple system that worked. The inverse proposition also appears to be true: a complex system designed from scratch never works and cannot be made to work You have to start over, beginning with a simple system.
-- John Gall, Systems Theorist
Complex systems are full of variables and interdependencies that must be arranged just right in order to function. Complex systems designed from scratch will never work in the real world, since they haven’t been subject to environmental selection forces while being designed. Uncertainty ensures that you will never be able to anticipate all of these interdependencies and variables in advance, so a complex system built from scratch will continually fail in all sorts of unexpected ways.