During the time of Hippocrates (~460 - ~ 377 BCE), the concept of body humors developed. Prior to this time, illnesses were attributed to one cause (e.g. beset by evil spirits) with varying symptoms. The four humors, or fluid substances, of the body, were blood, yellow bile, black bile, and phlegm. This theory was closely related to the theory of the four elements: earth, fire, water, and air. Earth was represented by black bile, fire by yellow bile, and water by phlegm. While the air was associated with blood, it was believed that all four elements were also represented by blood.
Blood was thought to be produced by the liver. Its temperament characteristics included courage, hopefulness, and playfulness. An imbalance of yellow bile (which was associated with the spleen) caused aggressiveness and excessive anger. Black bile was linked to the gall bladder. Its imbalance was thought to cause depression. In fact, the word melancholy is derived from the Greek word that means black bile. Apathetic behaviors were thought to be caused by an imbalance of phlegm. The brain and lungs were believed to be the source of this humor.
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