“Everything should be made as simple as possible, but not simpler.” – Albert Einstein
Simple means straightforward, uncomplicated, and direct.
Simple is easily unsterdood—“having few parts,” “having few properties,” unambiguous, clear, and easy to understand. Simple also has secondary meanings, including humble, not showy, unsophisticated, and guileless (incapable of deception)—or, in other words, “having a child-like mind,” the mind of a simpleton (a simple-minded person).
But something simple is not always “easy to understand”; simple things can still be hard to understand, such as space and time, as Einstein understood.
Simplistic means “inappropriately or deceptively simple”—for ideas, explanations, interpretations, and theories. A physical object, such as a box, can not be simplistic—because simplistic describes over-simplified representations of things. For example, simplistic often describes popular slogans or sayings, like ‘“Just say ‘No,’” a slogan that was once promoted to fight drug use in America. Unfortunately, stopping people from using drugs is not THAT simple! It was a simplistic slogan.
Simplistic ideas usually are simple, but not necessarily. For example, if you try to explain quantum field theory in five minutes, it would be simplistic, but not necessarily simple!