There are several synonyms for the word "rationalness," which is defined as the quality of being based on reason or logic. These include rationality, reasonableness, logic, soundness, coherence, intelligibility, and sensibleness. Rationality refers to the state of being based on reason or logic, while reasonableness implies that something is fair or sensible. Logic involves the principles of reasoning and inference, while soundness refers to the validity of an argument or reasoning. Coherence refers to the consistency or clarity of an idea or argument, while intelligibility implies that something is easily understood. Finally, sensibleness suggests that something is based on good judgment or common sense.