Yes, it’s MUCH easier to understand the first part of the above saying by Robert McCloskey.
Yet, I do like some negation cases! Especially double negation. It’s a fine way to hedge or downplay what you really think, like, “it’s not impossible,” “it’s not that I don’t like it…” Here, I do NOT refer to officialese or bureaucratese phrases like “not unless,” “not until” or “never without.” Or “don’t hesitate”:) They are “NO-NOs” (negation again?!)
Anyway, it is preferable to avoid negation (another negation case, which simply equals “use affirmative statements”:). Because positive sentences are so much easier to comprehend than negative ones. They are also easier to remember.
Affirmation and negation… Two sides of the same coin, actually.