The K.I.S.S. Principle
(Keep It Simple Stupid)
(Keep It Simple and Stupid), first coined by Kelly Johnson and so here are a few others with the same idea.

Things should be as simple as possible, but not simpler.
~ Albert Einstein

Most of the fundamental ideas of science are essentially simple, and may, as a rule, be expressed in a language comprehensible to everyone.
~ Albert Einstein

Making the simple complex is commonplace; making the complicated simple, awesomely simple, that's creativity.
~ Charles Mingus

Also by another Kelly: Nothing is as simple as it first seems.

Nothing is as simple as it looks: Murphy's Third Law

No endeavor that is worthwhile is simple in prospect; if it is right, it will be simple in retrospect.
~ Edward Teller

The more disastrous the mishaps the simpler the reviewing task.
~ Donal Henahan

To show our simple skill,
That is the true beginning of our end.
~ William Shakespeare

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Simple things amuse simple people.

--Professor Eskew, U.T.M. 1975

Our life is frittered away by detail; Simplify, simplify.
`Henry David Thoreau

If your mind isn't clouded by unnecessary things, This is the best season of your life.
~Wu-Men