Which is correct: simpler or more simple?

Many years ago I took a two-day course on Chicago Manual of Style. It was a great review of high school English classes and I learned much more than I expected. The lesson that stood out to me the most was that grammar and usage is, for the most part, a relative science. There are general rules, but the rules change and exceptions can be made.

An editor recently changed a sentence I had written in a manuscript from “more simple” to “simpler.” That seemed right to me. “Simpler” is just one wor...