When we say something "with reason", it implies a logical and sensible thought process behind our words or actions. There are several synonyms that can be used to convey the same meaning, including "justifiably", "rationally", "intelligently" and "wisely". "Cognizantly", "deliberately" and "prudently" also imply a thought-out or intentional approach to a situation. "Sensibly" and "judiciously" both suggest a measured and thoughtful response. Ultimately, any of these synonyms can be used to provide greater clarity to a statement or action that is based on a reasoned and sound foundation.