Phlegmatic

...Western words such as temperament, which means "to mix well," are derived from the Latin temperare, which means "to mix well." This is because, since Hippocrates, human personality types were thought of in terms of the mixture of four bodily fluids: blood, bile, black bile, and mucus. It was believed that if blood was predominant, a cheerful and lively "sanguine" personality would result, if bile was predominant, a short-tempered and excitable "choleric" personality would result, if black bile was predominant, a gloomy and melancholic "melancholic" personality would result, and if mucus was predominant, a dull and cold-blooded "phlegmatic" personality would result. Today, this so-called theory of the four elements (theory of the four humors) is nothing more than a product of fantasy lacking scientific evidence, but it was widespread not only in ancient Greece and Rome, when humoral pathology was considered the basis of medical science, but also through the Renaissance and into the 19th century, when modern medicine was established, and it may have had some influence on Kretschmer's theory of temperament, described below.

*Some of the terminology explanations that mention "phlegmatic" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…気質を表す英語temperamentなどの西欧語は語源的に〈ほどよく混ぜ合わせる〉という意味のラテン語temperareに由来しているが，これは，ヒッポクラテス以来，人間の性格類型を血液･胆汁･黒胆汁･粘液という4種の体液の混合のぐあいから考えたためである。このうち，血液が優勢ならば，陽気で快活な〈多血質sanguine〉が，胆汁が優勢ならば，短気で興奮しやすい〈胆汁質choleric〉が，黒胆汁が優勢ならば，陰気で憂鬱（ゆううつ）な〈黒胆汁質melancholic〉が，また粘液が優勢なら，鈍感で冷血な〈粘液質phlegmatic〉がそれぞれ生ずると信じられた。このいわゆる四性論(四体液説)は，こんにちの目からみれば，科学的根拠を欠く空想の産物にすぎないが，体液病理学を医学の基本とみなした古代ギリシア･ローマ期に広く行きわたったばかりか，ルネサンス期を経て，近代医学の成立する19世紀まで受け継がれ，下記のクレッチマーの気質論にも多少の影響をおよぼした節がある。…

※「phlegmatic」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報