The Doctrine of Signatures is a philosophy about plants that can be traced back to Medieval European alchemists, who believed that God gave plants a signature that hinted at its uses. Meaning, plants that resembled human body parts could be read as indicators of how those plants could be used to treat the body.

Popularized in the early 1600s by a German shoemaker who had mystical visions and wrote a philosophy book about the interconnectedness of nature and man, the Doctrine of Signatures was adapted by herbalists. They believed that because the leaves of a plant may have resembled a liver, say, they could be used to treat a bad one, or that an infusion of spotted leaves could help cure a diseased lung.

The Greeks prescribed brain-like walnuts to treat mental illness.