The words “proclivity” and “declivity” share the Latin root “clive,” which means “slope.” The prefixes “pro” and “de” can both mean “down,” but the words have different definitions. The prefix “de-” means “away,” which gives us the literal downward slope of “declivity.” The prefix “pro-” can refer to a physical motion indicating out or away, but it also implies something in favor of or in support of, which gives us the definition of “proclivity”: “to have a tendency toward.”