Natural magic was inseparable from what we’d consider “real” herbal lore. Since the chemistry of why some plants were medicinal was very, very far out of reach, “that’s just how its natural magic works” was the best explanation.
In pre-Christian times, natural magic didn’t cause any philosophical problems. In some religious systems, God is in everything, not separate from creation. In others, the plants are effective because using them is a supplication to some god to help. But for the early European Christians, the use of magic posed a dilemma. What was okay and what was not?
Most of what we know is from penance manuals, the books sent out into the field for priests to use at confession. Sins were carefully defined and, in a sense, rated or scored by how much penance was required. Calling on the pagan gods was obviously a sin, but many charms could be Christianized by calling on angels and saints, instead. I’ll have examples of these in future essays.
Using a plant in a direct way was never considered the wrong kind of natural magic. Plants were crushed, steeped, strained, and burned. They were mixed with wine, water, butter or other substances. Sometimes even animals were used for direct use in medicine, burning some or all of an animal and using the ashes. As long as the remedy went directly onto the wound or into the patient, it was fine.
The church was less comfortable with indirect use of plants, for example, to tie a bunch of leaves with a red string to the patient’s forehead. Some of the rules for using plants were right on the border, such as not using iron to cut or dig. Since chemistry was not understood (after all, using a copper or iron dish did make a difference in how a brew turned out), it was allowed. What about picking a plant only at dawn or midnight? Well, perhaps its virtues fluctuate with the day’s cycle. Who knows? When there was doubt, it was best to say the Our Father or invoke a saint, to be safe.
Everyone agreed on the basic principle of sympathy: similar appearance indicated that a plant (or, sometimes, animal) could treat a problem. If a plant’s flower looked like a snake’s head, it could probably cure snake venom. Garlic was good for spear wounds because the garlic bulb was shaped roughly like a spear-head (“gar” was spear in Anglo-Saxon). Jaundice was treated with plants that were yellow; tree bark could help with skin ailments.
Oddly, everyone also agreed that minerals had properties of natural magic. The lodestone pointed north, which was certainly natural magic, so why would not coral protect its wearer from lightning? Every gemstone had properties like this; one of the standard types of book listed the minerals and what they could do.
Thomas Aquinas, probably speaking for everyone in the Middle Ages, stated that all natural magic properties were caused by the stars’ influence on the earth. Astrology and astronomy were still one science in that time. Most of their lore came from the East, so it had a particularly exotic, scientific sheen.