Drawing on the works of ancient sources - most notably those brought together by the Roman physician Galen - the concept of the four humours was central to many aspects of Medieval life. 
In medicine, a proper balance of the humours (known as eucrasia ) was vital for purity and well-being. Illness directly stemmed from dyscrasia , a loss of this balance.
The four humours, black bile (responsible for melancholy), yellow bile (choler), blood and phlegm , gave rise to four temperaments: melancholic, choleric, sanguine and phlegmatic. Each of these reflected one of the four traditional elements of earth, air, fire and water. These tied into extremes of temperature and moistness , and are detailed in the diagram to the right.
A healthy individual sits squarely in the middle of the diagram. From the medical point of view, a loss of balance in the humours was commonly adjusted by surgeons of the period through blood-letting and the use of various emetics, to reduce the overwhelming humour which was responsible for that particular dyscrasia .