The “Theory of Signatures,” or “Doctrine of Signatures” is believed to have been started by a German shoemaker, Jacob Bohme, in the sixteenth century. To understand examples from the theory of signatures, first one must understand binomial nomenclature, the official naming system of living things.
Carl Linnaeus, a Swedish biologist created the binomial nomenclature system as it is currently employed. All living creatures, including plants, are classified into a taxonomic hierarchy, beginning with general groups (Kingdom) and ending with specific groups (species). The Sequence of plant classification from general to specific is: Kingdom, Phylum, Class, Order, Family, Genus, Species. The Genus and Species of a plant comprises its scientific name. Scientific names of plants (and all “identified” living creatures) are standard across all languages and cultures, and are in Latin. Plant names are in “Botanical Latin,” the rules of which are governed by the International Code of Botanical Nomenclature, most recently updated in 1994. Scientific names are always italicized. The first letter of the Genus is capitalized, and the first letter of the species name is in lower case. Example (minus the italics): Hepatica acutiloba, or commonly “liverwort.”
The Doctrine of Signatures
When Jacob Bohme published his book, “Signature Rerum,” (Signature of all Things) in the fifteenth century, it was a hit. In his book, Bohme described that all living things were marked with God’s signature, which indicated their usefulness. What began as one man’s belief or conjecture was soon accepted as valid science, and early botanists began naming newly discovered plants according to their “signature.” Hepatica acutiloba, mentioned above is one of the most commonly cited examples of a plant named according to the theory of signatures. Hepatica leaves have three lobes, like the liver, hence its name. Pulmonaria, or lungwort, has leaves with spots and blotches that look like the inside of a lung.
Because there was no scientific evidence, only strong belief, of the validity of the theory of signatures, plants were named and used with potentially harmful consequences. Sanguinaria canadensis, or bloodroot, is a spring wildflower with a root that, when crushed or broken, appears to “bleed” red sap. Genus name “Sanguinaria” comes from the Latin “sanguinarius,” which means bleeding. Sanguinaria was used to treat blood-related ailments. Unfortunately, the plant is poisonous to humans-something not known in the fifteenth century.
The theory of signatures, or doctrine of signatures is sometimes still applied in naming a newly categorized species, as far as the new scientific name might reflect a plant’s appearance. The “science” of the theory of signatures-using plants that look like a body part to treat an ailment with that body part-is not longer practiced. The theory is still alive, though, in the plants with 400 year old names, a living museum of relics of scientific practices past.