In this era of evidence-based medicine, where a herb is often only thought of as active if a single chemical substance responsible for its therapeutic effect is discovered or its efficacy is proven in a randomised controlled trial, traditional ways of healing are becoming forgotten.  

One example is the doctrine of signatures which traces back to antiquity. German mystic Jakob Böhme’s 1621 book, The Signature of All Things, gave its name to the doctrine. It is a 16th Century theory popularised by Paracelsus (1493 to 1541), considered the father of modern chemistry, who posited that plants had been signed by their Creator with visible clues to their usefulness. For example, a similarity in shape to various parts of the body meant the herb could be used to treat ailments of those parts of the body. 

Ginkgo (Ginkgo biloba) is often used as a fine example of the doctrine of signatures. Ginkgo’s signature is its curious fan shaped leaves with two hemispheres as the name biloba suggests. They resemble the outline of a brain, especially when grouped together. This, coupled with its long life, resistance to the environment and its being the world’s oldest living tree species, helps explain why this plant has been seen as a brain food for centuries. Eyebright’s (Euphrasia officinalis) signature is its striped flower petals, supposedly reminiscent of bloodshot eyes, and it is traditionally used for eye infections. Barberry (Berberis vulgaris), widely used as a cholagogue to treat liver and gallbladder ailments, has yellow flowers and roots which produce a yellow dye and these features were linked to the yellowing of the skin and eyes of jaundice, a symptom of liver disease. 

The doctrine of signatures is not supported by scientific evidence however recent research supports the therapeutic value of many signature-bearing species and validates their use in folk medicine. While it may be cast off as a fancified romantic notion the doctrine of signatures can, at the very least, be used as a learning aid to assist in remembering the materia medica and as a way to deepen the practice of herbal medicine. It is also a way of communing with nature and, in fact, the word signature is a mix between the words “sign” and “nature.”