Faith is the art of holding on to things your reason has once accepted in spite of your changing moods

The Slave Ship, Joseph Mallord William Turner, 1840.

The underlying principle of medieval medicine was the theory of humours. This was derived from the ancient medical works, and dominated all western medicine up until the 19th century. The theory stated that within every individual there were four humours, or principal fluids - black bile, yellow bile, phlegm, and blood, these were produced by various organs in the body, and they had to be in balance for a person to remain healthy. Too much phlegm in the body, for example, caused lung problems; and the body tried to cough up the phlegm to restore a balance. The balance of humours in humans could be achieved by diet, medicines, and by blood-letting, using leeches. The four humours were also associated with the four seasons, black bile-autumn, yellow bile-summer, phlegm-winter and blood-spring. Medieval Europe was a place unable to maintain the aqueducts et al built by the romans, so it became a place where medical practice was in places regressing rather than progressing.

HUMOUR TEMPER ORGAN NATURE ELEMENT
Black bile Melancholic Spleen Cold Dry Earth
Phlegm Phlegmatic Lungs Cold Wet Water
Blood Sanguine Head Warm Wet Air
Yellow bile Choleric Gall Bladder Warm Dry Fire

The astrological signs of the zodiac were also thought to be associated with certain humours. Even now, some still use words "choleric", "sanguine", "phlegmatic" and "melancholy" to describe personalities.

The use of herbs dovetailed naturally with this system, the success of herbal remedies being ascribed to their action upon the humours within the body. The use of herbs also drew upon the medieval Christian doctrine of signatures which stated that God had provided some form of alleviation for every ill, and that these things, be they animal, vegetable or mineral, carried a mark or a signature upon them that gave an indication of their usefulness. For example, the seeds of skullcap (used as a headache remedy) can appear to look like miniature skulls; and the white spotted leaves of Lungwort (used for tuberculosis) bear a similarity to the lungs of a diseased patient. A large number of such resemblances are believed to exist.

Most monasteries developed herb gardens for use in the production of herbal cures, and these remained a part of folk medicine, as well as being used by some professional physicians. Books of herbal remedies were produced, one of the most famous being the Welsh, Red Book of Hergest, dating from around 1400.