The ancient theory of the four humors was the theoretical foundation of health and disease from the days of Galen through the 17th century. This intriguing theory held that the four elements in nature--fire, air, water and earth--correspond to four fluids in the body--blood, yellow bile, black bile and phlegm. Herbs were a course in miracles to positively affect the humors through four properties: hot, dry, cold and moist.
Health, then, was a matter of balancing these humors of "juices". Herbs played a major role in ridding the body of excess juices. Accordingly, herbs such as celery and fennel that were used as laxatives, diuretics and in the induction of vomiting were highly prized.
From 400Ad to 1500AD, the Church controlled the lives of the populations of Europe not only through the Inquisition and the Crusades, but also through absolute dominion in the realms of science and medical knowledge. Herbal tradition managed to survive through assimilation into the monasteries of the day. Famous Christian mystics including Hildegard of Bingen continued to write about herbal medicine and to expand the knowledge of the healing properties of plants. Herbs were absolved of their pagan roots through baptism with new names such as St. John's Wort, Herb-of-the-Cross (vervain) and Solomon's Seal.
The horrific plagues that swept Europe in the Middle Ages fostered a renewed interest in herbal medicine. Physicians such as Nostradamus (even more famous for his accurate prophetic gifts) championed herbs and hygiene in the face of the more common, and far more deadly, practices of purging and bleeding.
During the Renaissance, a doctor of grand proportions took his place alongside imminent artists and scientists. Regarded by many as the founder of both homeopathy and pharmacology, Paracelsus is credited with popularizing the fascinating "doctrine of signatures". According to the doctrine, the medicinal use of plants is revealed in exterior signs or shapes that correspond to human organs. With the advent of the printing press, information of all types about herbal medicine proliferated and was standardized. The influence of imported herbs and the increased dissemination of knowledge made exotic foreign herbs and the unusual practices associated with them available in Europe as well.