(1)(能)推理的(having or using the ability to reason)：Man is a～animal. 人類是理性動物。Are men more～than women? 男人是不是比女人更有理性?
(2)講(合)理的，有理智的(reasonable;sensible;not influenced by emotion)：a～explanation/plan/suggestion 一個合理的解釋/計劃/建議;Her behaviour isn't～. 她的行為不理智。Panic destroys～thought. 驚恐使人喪失理智。〖同〗reasonable;
(3)頭腦清楚(健康)的(of sound mind)：A～man wouldn't do things like that. 一個頭腦正常的人不會干那樣的事。The old man was perfectly～when he made out his will. 那位老人立遺囑時頭腦很清楚?！纪絪ound;〖反〗 irrational;
【辨異】rational與reasonable都表示“講道理，有推理能力”。 rational表示比較抽象、純理性地推理，如：Her conclusions are always of a rational，never an emotional nature.(她的結論總是很理性的，從來不帶感情色彩。)reasonable(合情理的)表示簡單實際地推理，如：A reasonable supposition is one that appeals to our common sense.(所謂合理的推測就是迎合常識的推測。)