Occam’s Razor
Occam was a monk, circa 1280 1349
 
"Entia non sunt multiplicanda praeter necessitatum"
Create no more entities than are necessary.

This major design principle was expressed by an early modeler, and is most commonly expressed today as the KISS system; Keep It Simple - Silly.

Einstein restated this principle saying that solutions should be as simple as possible, and no simpler. The "and no simpler." is what brings home the difficulty in applying this principle. Although unfortunate, our solutions (designs) must meet their constraints; e.g., they must work.

In modeling this principle is restated: "Collapse the model as far as possible without losing integrity".

Another corrollary is: The higher the level of solution abstraction, the more effective and durable the design will be.

The balance to this principle is Caveat Emptor (Buyor Beware). In other words, the solution must work; its recipient must be able to use it, preferably with ease. This dichotomy gives rize to several other principles; e.g., the public/private model concept.