Simple - simplistic

Do not use the adjective simplistic under the mistaken impression that it means 'simple', nor the even further mistake of using it because it 'sounds better'.

• In present-day English, simplistic is properly used to mean 'too simple', 'treating a [complicated] matter with less respect for its complications than you should'. In other words, it is a pejorative word that carries a criticism in it.
• Simple should not be used pejoratively. It, together with the noun simplicity, naming 'the state of being simple', should be a neutral opposite of 'complicated' or 'complex'. Sometimes it can indeed be a compliment, as when a mathematician praises a colleague for "the simple elegance" or "the beautiful simplicity of his equations." It would be a great insult to call such an equation 'simplistic': this would imply that all the interesting bits had been left out, and that the writer had looked at a complex matter with the eyes of a child.

OED gives, as meaning 2. of simplistic: "Of the nature of, or characterized by, (extreme) simplicity. Now usu[ally] with the connotation of excessive or misleading simplification," citing the illustrative quotation "The facts of nature and of life are more apt to be complex than simple. Simplistic theories are generally one-sided and partial" (Clarke, James F., Self-culture, 1880. This quotation illustrates the point of this article in AWE rather neatly.) (Meaning 2. is the obsolete 'to do with simples, or medicinal herbs: there is a 'now rare' word simplist, meaning a herbalist; 'one who studies medicinal plants'.)