Not unreasonable: Why two negatives don't make a positive
Logic tells us that two negatives make a positive, but in language, things are not so black and white: A person "not unhappy" may not be entirely happy. We hypothesize that innovative uses of double negatives like "not unhappy" stem from listeners entertaining flexible meanings for negation markers like "not" and "un-", which context can then help disambiguate. We formalize this hypothesis in a computational model of language understanding, which predicts that "not unhappy" means something different from "happy" but also makes the additional prediction that single negations ("unhappy" vs. "not happy") are interpreted identically except when a speaker uses both in the same context, which we confirm experimentally. Even double negations that flagrantly use the same negation marker twice (e.g., "not not happy") are interpreted in subtle ways. These findings suggest that even one of the most logical elements of language—negation—can mean many things at once.