A woodcut by 15th century Dutch artist Erhard Reuwich of a doctor holding a container filled with a patient's urine. In medieval medicine, uroscopy was an important diagnostic tool, urine having 29 attributes to be observed. From this the physician deduced the supposed levels of a patient's four 'humors' (bodily fluids): blood, yellow bile, black bile and phlegm. Illness was thought to be due to an imbalance in these fluids. It was thought that the imbalance could be redressed by changing the patient's diet, prescribing medicine, by bleeding the patient or by surgery. This woodcut c. 1480s.