Gall's Law is a rule of thumb from John Gall's Systemantics: How Systems Really Work and How They Fail:
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 working simple system.
Systemantics (a change in typography and underlining indicate that the title is better rendered as "SystemANTICS") is a commentary on systems theory and general semantics publications by such thinkers as Ludwig von Bertalanffy and Alfred Korzybski.
Although dubbed Gall's Law by some, the actual quote is not labeled as such in the original work. The work cites Murphy's Law and the Peter Principle and is filled with similar sayings.
Although the quote may seem to validate the merits of simple systems, it is preceded by the qualifier "A simple system may or may not work." (p. 70).
This philosophy can also be attributed to Extreme Programming, which encourages doing the simplest thing first and adding functionality later.
See also