The doctrine of signatures is the belief that plants bear the "signature" of what they are useful for by sharing a resemblance with the organ they are meant to treat.
This doctrine was subscribed to and taught for centuries from the ancient Greeks to Medieval physicians. Renaissance physician and alchemist Paracelsus first codified the concept saying "Nature marks each growth … according to its curative benefit".
Some gave credit to God rather than nature, as William Coles (1600s) stated The mercy of God… maketh… Herbes for the use of men, and hath… given them particular Signatures, whereby a man may read… the use of them. A wise choice of words in an era when witches were unpopular and herbalists were suspect. William Coles believed that walnuts were good for the head, because they look like a head, and brains and that St John's Wort would be soothing to the skin because it had pores like skin. In both cases, he was correct.
Many common names of plants bear the memory of the doctrine of signatures. The flowers of eyebright were said to resemble eyes, and to be useful for ailments of the eyes. The leaves of lungwort, which bears the modern botanical name Pulmonaria (referencing the pulmonary system, i.e. lungs) resemble lungs and were believed to treat diseases of the lungs.
The doctrine of signatures was not limited to Western healing but also existed and continues to exist in Eastern tradition. In a somewhat more complicated tradition, Chinese medicine corresponds colors and flavors in plants to the organs they are used to treat.
While there is no scientific evidence to back up the doctrine of signatures, there have been many coincidences.
- The Doctrine of Signatures a PDF from Portland Herbal School
- The Doctrine of Signatures from Economic Botany at UCLA
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