This essay is a draft and will expand/change over time.
The way that oils and herbs are chosen for inclusion in a ritual oil is by using a table of correspondence that’s based on sympathetic magic. One important component of sympathetic magic that involves plants is the doctrine of signatures, which was focused on medicinal practice during the medieval to early modern era. The doctrine of signatures was expanded to include metaphysical attributes of parts of the natural world.
The original idea in the doctrine of signatures was that injuries, diseases and medical conditions could be treated or healed by plants, and that the plants could be discovered by their resemblance to the body part it treated, or by the disease it cured. This was an important part of folk medicine from the middle ages until early modern times – the common names of plants often reflect this thinking, where the use of a plant depended on its association with the body or with a disease. The Doctrine of Signatures was promoted in the West by the Swiss physician Paracelsus and later named and popularized by Jakob Böhme in 1621 with his book Signatura Rerum (The Signature of All Things). Böhme believed that the Doctrine of Signatures had two main component:
“A) Nature has provided in every region plants for remedies which the diseases common to that region required for cure or treatment.
B) Nature has also provided signs or symbols to indicate by physical characteristics of the drug (color, shape, taste, etc.) the diseases/disorders for which the plant is a remedy.”
Hocking, Geo. M. “The Doctrine of Signatures.” Quarterly Journal of Crude Drug Research15, no. 4 (1977): 198–200. https://doi.org/https://doi.org/10.3109/13880207709055172.
From our current perspective, we can see that much of the success attributed to the doctrine of signatures was attributions made after the fact, or by using the functional plant names as a mnemonic tool to remember the actions of medicinal herbs. At the time, however, both disease and the treatment of disease were seen as mystical in nature, if not divine. Nature – or, later, the Christian God – had given us the tools to heal the human body, it was believed. Humans simply needed to use the the existing tools correctly, each herb or plant or item in the right way to treat the right problem. The Law of Similarity was used for medical purposes, to connect plants to humans, with new correspondences developing as we discovered uses for herbs and plants. It wasn’t very effective as a medical practice, but an excellent illustration of how humans reach for connection, how we try to create analogies from one sphere to another.