Signatuurleer / Doctrine of signatures

Doctrine of Signatures: Through Two Millennia

HerbalGram. 2008; :34-45 American Botanical Council

Abstract. The Doctrine of Signatures is widely cited in the literature on medicinal plants. According to the theory, certain physical attributes of plants serve as signs to indicate their therapeutic value. Pliny the Elder, Dioscorides, and other early classical scholars allude to the theory, but it was best developed by Paracelsus and his followers during the Middle Ages. German religious mystic Jakob Böhme and English herbalists Nicholas Culpeper and William Cole were among its strongest proponents. Beginning in the mid 1500s, scholars began to criticize the notion of signatures. Flemish physician and herbalist Rembert Dodoens was perhaps the first to challenge its validity. English naturalist John Ray also was critical of the theory. Modern scholars are nearly universal in discounting the Doctrine of Signatures, calling it absurd, fanciful, far-fetched, and pseudo-scientific. Nonetheless, researchers continue to refer to the doctrine as the reason that many plants are selected for medicinal use. Careful evaluation of the Doctrine of Signatures shows that it did not function as an a priori clue to therapeutic value. Instead, it served as a mnemonic, which was especially important in preliterate cultures.


The Doctrine of Signatures (DOS) is a widely cited theory that purportedly explains how humans discovered the medicinal uses of some plants.1,2 According to DOS, physical characteristics of plants (including shape, color, texture, and smell) reveal their therapeutic value. R.H. True, a plant physiologist and historian, succinctly explained the doctrine: "… every plant having useful medicinal properties bears somewhere about it the likeness of the organ or of the part of the body upon which it exerts a healing action."3 Examples that fit this classical morphological definition include the coiled roots of Indian snakeroot (Rauvolfia serpentina, Apocynaceae) for snakebite,4 the clinging fruits of wild comfrey (Cynoglossum virginianum, Boraginaceae) to improve memory,5 and the phallic-looking roots of mandrake (Mandragora officina-rum, Solanaceae) for masculine diseases.6

DOS also has a more specific meaning. Some believe that the Creator gave physical clues about the value he imbued to plants. In 1669 Oswaldus Crollius wrote: All herbs, flowers, trees, and other things which proceed out of the Earth, are books, and magick signs, communicated to us, by the immense mercy of God, which signs are our medicine. … for every thing that is intrinsic, bares the external figure of its occult property ...7

DOS is ubiquitous. Anthropologist William Balée and others have suggested that it is a near universal phenomena.8 According to the 19th century surveyor of the Grand Canyon area, J.W. Powell, "All American tribes entertain a profound belief in the doctrine of signatures …"9 B.E. Read, an early 20th century pharmacologist, wrote that DOS is "to be found extensively quoted in the Orient and Occident."10 It is frequent in medieval botanical texts "owing to the widespread belief in the doctrine of signatures."1 DOS is alluded to in classical Greek literature on medicinal plants. Elements of the doctrine are present in the writings of Hippocrates, though he also advocates the principle that opposites are cured by their opposites-the antithesis of DOS.11 Furthermore, Fielding Garrison argues that with respect to animal based medicines, Greek medicine did not employ the principle that likes are cured by their likes.12 DOS appears in the English Restoration literature. In Paradise Lost, the 17th century English poet John Milton described the Archangel Michael's use of eyebright (Euphrasia officinalis, Orobanchaceae) to restore Adam's vision.13 Eyebright's signature is its striped petals, supposedly reminiscent of bloodshot eyes.14 DOS appears in current literature, as well. In Harry Potter and the Chamber of Secrets, Madam Pomfrey revives petrified students with a potion made from mandrake (M. officinarum) roots.15 Mandrake's signature is found in its roots, which resemble the human body.16 Other examples of DOS appear in Table 1 on pages 38-39.

European herbalists used the yellow latex of celandine (Chelidonium majus, Papaveraceae) to cure liver ailments. In her account of celandine, the 20th century English herbalist and author Maude Grieve wrote, "The old alchemists held that it was good to ‘super-stifle the jaundice,' because of its intense yellow colour."17 The Quichua of lowland Ecuador alleviate menstrual bleeding with the red-tipped leaves of Columnea ericae (Geseneriaceae) [BC Bennett, unpublished field notes]. Chinese esteem ginseng (Panax ginseng, Araliaceae) roots because of their human-like rhizomes.18 The medieval Swiss chemist and physician Paracelsus declared, "Flowers that are of a burning color like the rose (Rosa spp., Rosaceae) are apt to heal inflammations; those which bear the color of a face heated by wine, as the rose does, obviates drunkenness."19

According to 17th century English physician Nicholas Culpeper, who compiled an extensive list of medicinal plants, the aptly named liverwort "… is an excellent remedy for such whose Livers are corrupted by surfets which causeth their bodies to break out, for it fortifies the Liver exceedingly and makes it impregnable."20 (The botanical identification of Culpeper's liverwort is uncertain. Possible candidates include Marchantia polymorphia, Marchantiaceae; Peltigera canina, Peltigeraceae; and Hepatica nobilis, Ranunculaceae.)

William Cole, England's foremost DOS exponent, wrote: Wall-nuts have the perfect Signature of the Head: The outer husk or green Covering, represent the Pericranium, or outward skin of the skull, whereon the hair groweth, and therefore salt made of those husks or barks, are exceeding good for wounds in the head. The inner wooddy shell hath the Signature of the Skull, and the little yellow skin, or Peel, that covereth the Kernell, of the hard Meninga and Pia-mater, which are the thin scarfes that envelope the brain. The Kernel hath the very figure of the Brain, and therefore it is very profitable for the Brain, and resists poysons; For if the Kernel be bruised, and moystned with the quintessence of Wine, and laid upon the Crown of the Head, it comforts the brain and head mightily.21

Reverend Edward Stone reportedly believed that English willow (Salix spp., Salicaceae), a tree that thrived in moist environments, could cure rheumatism, which was associated with similar environments.22 Plants that resembled semen or human sexual organs, were considered to be aphrodisiacs. Cucumbers (Cucumis sativus, Curcurbitaceae) and carrots (Daucus carota, Apiaceae) were employed in China and Korea to increase sexual potency.23 The red signature of beet (Beta vulgaris, Amaranthaceae) root affirmed its use in China to treat dysmenorrhea.10

Plants that exude gums were utilized in treatments for purulent conditions. Quaking aspen (Populus tremuloides, Salicaceae) leaves were employed to treat palsy. Evergreens and long-lived plants supposedly increased longevity. Heliotrope (Heliotropium spp., Boraginaceae) and marigold (Calendula officinalis, Asteraceae) were prescribed so that subjects might "learn their duty to their sovereign." Both plants are phototropic. King Charles quipped that "the marigold observes the sun more than my subjects have done."24

History of the Doctrine of Signatures

Pliny the Elder (23-79 CE) According to some authorities, Pliny the Elder's Natural History contains one of the earliest references to DOS.25 However, Pliny is ambiguous in his support of DOS. Furthermore, it is unclear whether he is advocating signatures or simply restating the opinions of others. It is likely that he would not have ascribed to any such theory because of his Stoic beliefs, yet allusions to signatures do appear in his writings. For example, Pliny writes, "A hot decoction of rape [Brassica napus or B. rapa, Brassicaceae*] is employed for the cure of cold gout."26 However, this citation follows Galen's contraria contrariis curantur (opposites are cured by their opposites) philosophy more than it reflects a signature. Bellavite et al cited Pliny's claim that the saliva of a rabid dog affords protection against rabies as evidence of his support of DOS.27 However, Pliny mentions many other cures for rabies that do not invoke DOS.

Pliny also offers reasons other than DOS to explain the use of plants. For example, he wrote, "The adiantum is of singular efficacy in expelling and breaking calculi of the bladder, the dark kind in particular; and it is for this reason, in my opinion, rather than because it grows upon stones, that it has received from the people of our country its name of saxifragum."26

The name saxifragum, and the genus Saxifraga (family: Saxifragaceae), are derived from the Latin roots saxum (rock) and frangere (to break up).† A plant growing in the cracks of a rock, whose roots were splitting or widening the rock, was thought to have "stonebreaking" qualities, and therefore good for bladder and kidney stones.26

Pedanius Dioscorides (circa 40-90 CE) Dioscorides, a Greek physician and pharmacologist, traveled widely during his tenure as a surgeon with Roman armies. He is best known for his extensive De Materia Medica, which endured as the standard pharmacology text for about sixteen hundred years. Dioscorides is seldom mentioned in discussions of the DOS, most likely because he did not accept it as a theoretical basis for medicinal plant selection. Nevertheless, when reporting conventional usages, occasionally he included evocations of DOS.

The Italian scholar Giambattista Della Porta wrote, "Dioscorides says, that the drugs which grow in steep places, cold and dry, and open to the wind, are most forcible, but they that grow in dark, and waterish, and calm places, are less operative."28 While this statement could loosely be interpreted to represent DOS, it is more likely a reflection of Dioscorides' concern for variation in the quality of herbal medicines. Della Porta cites another quote from De Materia Medica that could be interpreted to support DOS: "Dioscorides writes, that the Herb Scorpius resembles the tail of the Scorpion, and is used against his biting."29 However, the use of a conjunction between the signature and the plant's use, rather than a conjunctive adverb is significant. The phrase "and is used against his biting" is quite different from "and therefore is used against his biting." The implicit inclusion of the conjunctive adverb "therefore" creates the logical fallacy of post hoc ergo propter hoc ("after this, therefore because of this"), which is common in interpretations of signatures. Dioscorides' description of balananon elaion (oak galls, probably from Quercus lusitanica or Q. infectoria, Fagaceae) to "stop abnormal growths of the flesh,"29 is also interpreted by some to support DOS.‡

Galen (131-201 CE) Claudius Galenus, or Galen, ascribed to Hippocrates the theory that health required the maintenance of equilibrium among the various humors, e.g., blood, yellow bile, black bile, and phlegm. Some scholars claim that he was one of the earliest proponents of DOS, but they would be incorrect. One can see the theory in operation in the earliest pharmaceutical works.

Maycock noted, "Galen was resolute in his confidence of an underlying design throughout nature to be demonstrable in accord with a doctrine of signatures."30 However, evidence for Galen's adherence to DOS is lacking. Paracelsus, the greatest DOS proponent, rejected Galen and other classical authorities by publicly burning their texts. Galen is perhaps best known for the concept contraria contrariis curantur (the opposite is cured with the opposite), the foundation of so-called allopathic medicine. Accordingly, heat would be applied for diseases rooted in coldness and vice versa.31

Paracelsus (1493-1541) Paracelsus, or Philippus Theophrastus Aureolus Bombastus von Hohenheim, served as the municipal physician of Basel and as a lecturer at the city's university. He opposed Galenic medicine, promoting an experimental approach instead.32 Paracelsus has been called the "father of chemistry," "founder of medicinal chemistry," and "father of toxicology." The latter title derives from his proclamation that, "Solely the dose determines that a thing is not a poison."33 The statement reveals far more than the founding principle of toxicology. It reveals Paracelsus' faith in the efficacy of inorganic substances in medicine, despite his critics' assertions that they were too toxic to be used as therapeutic agents.33 The therapeutic use of purified chemicals (like arsenic, lead, mercury, and any single inorganic compound), specifically inorganic elements, was a 16th century innovation of Paracelsus.34

Paracelsus and his followers espoused the principle similia similibus curantur (like cures like).35 In Paracelsian medicine, treatment was directed against the external agent of disease, rather than toward the restoration of a balance in the humors.36 Paracelsus believed that plants, animals, and minerals were put on Earth by the Creator for human use. God provided signs within plants to indicate their uses.

"Nature marks each growth which comes from her according to its curative benefit … and there is no thing in nature, created or born, which does not strive to reveal its inner form outwardly; for the inner life continually works toward revelation."37

Nicolas Bautista Monardes (AD 1493-1588) Monardes, the most famous Spanish physician of his time, practiced in Seville. He published a collection of his writings in 1574, which included a description of the newly introduced South American plant, coca (Erythroxylum coca, Erythroxylaceae) and its uses.38 Monardes was among the first Europeans to promote medicinal plants from the New World.39 Though he never saw the New World, as a proponent of DOS, Monardes accepted Native American botanical remedies, though he sometimes modified them.40

Giambattista Della Porta (1538-1615) A disciple of Paracelsus, Della Porta interpreted the virtues of plants based on their physical characteristics. His writings in Magia Naturalis and Phytognomonica were considered the definitive DOS exposition of the 16th and 17th centuries41 and he developed the theory in great detail.42 Della Porta's support of DOS is unequivocal. For instance, he wrote "… Bugloss and Orchanet bear seeds like a Viper's head, and these are good to heal their venomous bitings. Likewise Stone-crop and Saxifrage are good to break the stone in a man's bladder."43

Oswaldus Crollius (circa 1560-1609) Another follower of Paracelsus, Oswald Croll (or Oswaldus Crollius) opposed the Galenic theory of humors and sought external causes of disease.36 Croll's support of DOS is evident in his De Signaturis Internis Rerum (Treatise of Signatures of Internal Things), first published in 1669. According to Croll, the woody scales of pine cone resemble the fore-teeth; therefore pine leaves boiled in vinegar were used for the relief of toothache.44 Croll is adamant in his espousal of DOS as the key to elucidating the value of medicinal plants.

"In like manner, herbs magically by their signature bespeak the physician's thorough introspection, and to him by similitude manifest their interiors, concealed in the occult silence of Nature."45

Jakob Böhme (1575-1624) Jakob Böhme, a German religious mystic, was a master cobbler by profession. He experienced a mystical vision in 1600, which revealed to him that the relationship between God and man was signaled in all creation.46 In 1621, he published Signatura Rerum (The Signature of all Things). Here, Böhme reiterated the Paracelsian theory that the inner qualities and properties of all things are displayed in their outer forms. He advised all men to study nature with this in mind, assuring them that, "the greatest understanding lies in the signatures, wherein man may not only learn to know himself, but also the essence of all essences."47 Though his interests were primarily mystical and spiritual, his concept of signatures extended to medicinal plants.

"And there is nothing that is created or born in nature, but it also manifests its internal form externally, for the internal continually labours or works itself forth to manifestation … which we see and know in the stars and elements, likewise in the living creatures, and also in the trees and herbs."48

Nicholas Culpeper (1616-1654) Culpeper, the English physician, herbalist, and astrologer, believed that only astrologers were fit to study medicine. He described the College of Physicians in London as, "A company of proud, insulting, domineering doctors, whose wits were born about 500 years before themselves."49 Culpeper's herbal, first published as The English Physitian (1652), has been continuously published for more than 350 years. His works targeted the popular audience and included explicit references to DOS and to astrological theories as well.50 Culpeper valued signatures in plants, as clues to their medicinal value. He wrote "… by the icon or image of every herb, man first found out their virtues. Modern writers laugh at them for it, but I wonder in my heart how the virtues of herbs first came to be known, if not by their signatures. The moderns have them from the writings of the ancients-the ancients had no writings to have them from."49

William Cole (1626-1662) The 17th century botanist William Cole (sometimes misspelled as Coles) is known for his two books, The Art of Simpling (1656) and Adam in Eden (1657). He was the major English proponent of DOS and, like his contemporary Culpeper, wrote for the populace. Cole, however, criticized Culpeper, describing him as "ignorant in the forme of Simples."51

He was also critical of Culpeper's incorporation of astrology into herbal practices.52 Cole believed that some plants were given signatures "in order to set man on the right track," but others were left blank to encourage humans to discover them.42

For Cole, glands in the leaves of St. John's wort (Hypericum perforatum, Clusiaceae), which resembled skin, signified the use of the plant to treat skin problems.53 He argued that walnuts were efficacious in curing head ailments because, "They have the perfect signatures of the head." Cole's Art of Simpling is still in press and quoted widely in many herbals. Here, DOS is emphatically promoted.

"Though Sin and Sathan have plunged mankinde into an Ocean of Infirmaties, yet the mercy of God, which is over all His workes, maketh Grasse to grow upon the Mountaines, and Herbes for the use of men, and hath not only stamped upon them a distinct forme, but also given them particular Signatures, whereby a man may read, even in legible characters, the use of them."21

Recent History The pansemiotic view of the environment, including DOS, reached its apex during the Renaissance.54 Paracelsian physicians of the 16th century showed a renewed interest in DOS, arguing that their predecessors had misunderstood the theory. They expanded the concept of signatures to include chemical clues.55 Panese argued that DOS held significant influence in science and medicine from the 16th century onwards.56 In contrast, Maclean claimed," few non-Paracelsian doctors find anything to recommend in the doctrine of plant signatures."57

The modern literature continues to allude to DOS. Five publications in the Index to Economic Botany (1947-1996) refer to the concept.58 More recently Dafni and Lev described DOS in modern Israeli folk medicine.59 Publications in other journals also refer to DOS. Davis and Yost discussed the Ecuadorian Waorani's sympathetic magic to guide their healing practices.60 They noted that the use of plants with strong odors to counter symptoms is reminiscent of DOS. Schultes and Hoffmann suggested that the European fear of mandrake during the Middle Ages was due to DOS.61

Critics of the Doctrine of Signatures

Rembert Dodoens (1517-1585) In 1583, the Flemish physician and herbalist Rembert Dodoens declared that "[DOS] has received the authority of no one ancient writer who is held in any esteem: moreover it is so changeable and uncertain that, as far as a science of learning is concerned, it seems absolutely unworthy of acceptance."42

John Ray (1627-circa 1705) Ray, an English philosopher, theologian, and naturalist ranks among the most influential British natural historians of his era.62 According to Thiselton-Dyer, Ray, in his treatise The Wisdom of God Manifest in the Works of Creation (1691), was among the first to express disbelief in DOS, though he was probably not entirely free of its influence.44 Ray described his skepticism regarding DOS in his 1660 Cambridge Catalogue.63

"As for the signatures of plants, or the notes impressed upon them as notices of their virtues, some lay great stress upon them, accounting them strong arguments to prove that some understanding principle is the highest original of the work of Nature, as indeed they were could it be certainly made to appear that there were such marks designedly set upon them, because all that I find mentioned by authors seem to be rather fancied by men than designed by Nature to signify, or point out, any such virtues, or qualities, as they would make us believe."64

Samuel Hahnemann (1755-1843) In 1810, Hahnemann, a German physician, published Organon of Rational Therapeutics, which outlined the principles of homeopathy. Hahnemann was justifiably dissatisfied with the heroic medical practices (bleeding, purging, vomiting, etc.) and the materia medica of his times: "From the earliest beginnings until now, the materia medica has consisted only of false suppositions and fancies, which is as good as no materia medica at all."65 In contrast to the contraria contrariis curantur principle of Hippocrates and others, he advocated the principle of similia similibus curantur or like cures like.66 Fishbein, however, claimed that Hahnemann's thesis was a revival of the Paracelsian Doctrine of Signatures, except that Paracelsus focused on causes of disease rather than their symptoms.35

Richardson-Boedler concluded that Hahnemann did not utilize DOS.37 In fact, Hahnemann clearly rejected it: "The…virtues of medicines cannot be apprehended by… smell, taste, or appearance…or from chemical analysis, or by treating one or more of them in a mixture…"65 His contempt for DOS is without question.

"I shall spare the ordinary medical school the humiliation of reminding it of the folly of those ancient physicians who, determining the medicinal powers of crude drugs from their signatures, that is, from their colour and form, gave the testicle-shape orchis-root in order to restore manly vigour, the phallus impudicus to restore weak erections, and considered Hypericum perforatum, whose yellow flowers on being crushed yield a red juice (St. John's blood), useful in haemorrhages and wounds, etc.; but I shall refrain from taunting the physicians of the present day with this absurdity, although traces of it are to be met with the most modern treatises on Materia Medica." 67

T. F. Thiselton-Dyer (1848-1928) Thomas Firminger Thiselton-Dyer's Folk-Lore of Plants (1889) examined DOS in the 16th and 17th centuries. This is perhaps the most comprehensive treatment of the theory to date. The folklorist found reference to DOS in most medical works, noting that it was "treated with a seriousness characteristic of the backward state of medical science even at a period so comparatively recent." According to Thiselton-Dyer, DOS "led to serious errors in practice."34

James Mooney (1861-1921) In reference to the Cherokee, Mooney first denigrated traditional medicinal knowledge: "…their theory and diagnosis are entirely wrong, and consequently we can hardly expect their therapeutic system to be correct."68 Though admitting that many plants in their pharmacopoeia, "possess real curative properties" he added, "Thus at the present day the doctor puts into the decoction intended as a vermifuge some of the red fleshy stalks of the common purslane or chickweed (Portulaca oleracea, Portulacaceae), because these stalks somewhat resemble worms and consequently must have some occult influence over worms."

Recent History Read suggested that the influence of DOS diminished beginning in the 1700s,10 but Arber argued that it persisted into the 19th century. She cited Thomas Green's Universal Herbal published in 1816: "Nature has, in this country, as well as in all others, provided, in the herbs of its own growth, the remedies for the several diseases to which it is most subject."42

Modern scholars are near universal in their condemnation of DOS. The theory is called absurd, fanciful, far-fetched, discredited, premodern, prescientific, primitive, unreliable, and unscientific. Simpson and Ogorzaly averred that the idea "seems absurd now but received great acclaim when it was proposed. Luckily, it was soon displaced by less subjective and more secular methods of determining a plant's medical efficacy."69 Tyler ranked it among his False Tenets of Paraherbalism.70 Reed claimed that the theory has been completely debunked.71 Barfod and Kvist concluded, plants "used according to the Doctrine of Signatures" are less promising as pharmacological leads than other medicinal plants.72

A New Perspective on the Doctrine of Signatures

The late Harvard University biologist and popular author Stephen Jay Gould provided the most objective assessment of DOS, noting that the theory represents the key difference between modern science and an older view of nature.

"I question our usual dismissal of this older approach as absurd, mystical, or even prescientific (in any more than a purely chronological sense). Yes, anointing the wound as well as the weapon can only be labeled ridiculous mumbo-jumbo in light of later scientific knowledge. But how can we blame our forebearers for not knowing what later generations would discover? We might as well despise ourselves because our grandchildren will, no doubt, understand the world in a different way."36

Is DOS nothing more than "ridiculous mumbo-jumbo"? I recently proposed that DOS be reevaluated based on 4 criteria: (1) Its role in the discovery of medicinal plants, (2) Post hoc attribution of signatures, (3) The nature of signatures, and (4) Its role as a mnemonic. There is no evidence that DOS was used to discover the utility of medicinal plants. Researchers were not present when plants were first incorporated into pharmacopeias. Moreover, many plants with apparent signatures are not utilized while others lacking a correlated sign are employed.73

The Hermetic literature of late antiquity and medieval periods was widely read in Greek, Syriac, Arabic, and Latin and tended to emphasize that "God has endowed each herb, each stone, each star, and each sign, with a ‘secret' which, when it becomes known to man, will be of utility." The authors of this literature generally recorded secondhand accounts of healing; they were not practitioners or even discovers of medicinal plant knowledge.

Signatures are post hoc attributions rather than a priori clues. This contradicts the prevailing wisdom on DOS. Coulter, for example, wrote, "The doctrine of signatures and the microcosm-macrocosm correspondences seem to provide the physician with a priori knowledge of the remedies."74 For example, Read wrote, "… in some cases the signatures of drugs were observed after their real use had been discovered."10

No evidence supports the thesis that signatures were used to discover medicinal attributes of plants. Nonetheless, recent research supports the therapeutic value of many signature-bearing species. For example, Mooney's dismissal of purslane (Portulaca oleracea, Portulacaceae) was premature. Recent studies have shown that the plant is effective for controlling intestinal parasite loads and that it has gastroprotective action,75,76 which validates its use in folk medicine for gastrointestinal diseases. Instead of a clue that led to its discovery, a more parsimonious interpretation would be that the resemblance to worms aided in transmission of knowledge about the plant's use. Similarly, bitter melon (Momordica charantia, Cucurbitaceae) is consumed throughout the Caribbean as a blood tonic.77 The plant's seeds have a bright red aril and healers associate this signature with blood. Extracts from the plant normalize blood glucose levels, reduce triglyceride and LDL levels, and increase HDL levels.78 Even walnuts (Juglans regia, Juglandaceae), with their often-ridiculed cotyledon signature of a human brain, may be effective in treating cerebral ailments. Melatonin occurs in walnuts. When they are consumed by rats, blood melatonin concentrations increase.79 Melatonin is effective in relieving a variety of brain-related problems in laboratory animals including inflammation associated with cerebral ischemia.80, 81

The notion of signatures should be expanded to include non-morphological signatures. Strong odors in plants are correlated with the presence of volatile compounds, most of which are biologically active. Debus was among the first to discuss this expanded concept of signatures, writing, "The Paracelsian chemical physicians … disagreed intensely with those who sought to understand and to identify them only through their external shape or appearance. The chemist's own laboratory procedures seemed to offer the proper key to nature's hidden secrets."55 Davis and Yost's suggestion that plants with a strong odor might repel symptoms echoed a similar view.60

Most importantly, DOS served as a mnemonic, an idea first alluded to by Buchanan in 1938.50 The Doctrine of Signatures is primarily a way of remembering and transmitting plant knowledge, not a means of discovery. Post hoc attribution of signatures served as a memory aid that was particularly useful in preliterate societies. A 1996 study concluded that plants "used according to the Doctrine of Signatures" are less promising pharmacological leads than other medicinal plants. There is no basis for this assertion as many signature-bearing plants have proven to be efficacious in treating conditions suggested by their signatures.82


The Doctrine of Signatures is ubiquitous in the literature on medicinal plants. According to most proponents of the theory, the Creator imbued healing plants with a sign that revealed their therapeutic value. Early classical scholars, including Pliny and Dioscorides, allude to DOS, but the theory was best developed by Paracelsus and his students in the Middle Ages. The German religious mystic Jakob Böhme along with the English herbalists Nicholas Culpeper and William Cole were among the strongest DOS proponents. Beginning in the mid-1500s, scholars began to criticize the notion of signatures. Flemish physician and herbalist Rembert Dodoens was first to challenge DOS's supremacy. The English naturalist John Ray also was critical of the theory. Modern scholars are near universal in discounting the theory. However, a careful evaluation of DOS suggests that its value as a mnemonic, rather than an a priori clue to value, should not be dismissed.

Bradley C. Bennett, PhD, is an associate professor (within the Department of Biological Sciences) and director of the Center for Ethnobiology and Natural Products at Florida International University. His research focuses on Neotropical ethnobotany. He has published one book, 55 papers, 7 book chapters, and 40 book reviews. He was the 2004-2005 president of the Society for Economic Botany, and he currently serves as an associate editor of Economic Botany and a member of the ABC Advisory Board. He also serves frequently on National Institute of Health (NIH) and National Center for Complementary and Alternative Medicine (NCCAM) review panels. Dr. Bennett's Ethnobotany of the Shuar of Eastern Ecuador (New York Botanical Garden Press, 2002) won the 2006 Klinger Book Award.

Contact information: Dept. Biological Sciences, Florida International University, Miami, FL 33199. Phone: 305-348-3586 e-mail:

‡ Another example of Dioscorides possibly alluding to DOS is regarding the orchid. Dioscorides (/De materia medica./ 3. 126, Beck transl. 2005, p. 237) re. /Orchis /(Orchis papilionacea + spp.) wrote the following: "It has a bulbous root that is somewhat long, has two parts, and it is narrow like an-olive. One part is high up, the other lower down, one is full the other soft and shriveled. The root is eaten boiled like a bulb. About this plant, too, it is said that men who eat the larger root sire males, and that women eating the smaller give birth to females. And they say that women in Thessaly drink its soft growth with goat's milk to arouse sexual desires, and the dry one to check and abate them, and that the activity of the one is cancelled [sic] by the activity of that which is drunk afterwards." It is very diffi-cult to find indications of magic and DOS in Dioscorides but this is clear. The Greek word orchisalso means "testicles." Note, however, Dioscorides distances himself from the claims of its action by writing "it is said..." and "And they say...." He did this so that he did not have to stand by the claim.


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The Strange History of Using Organ-Shaped Plants to Treat Disease / Matt Simon

For tens of thousands of years before modern medicine, choosing plants that not only wouldn’t kill you, but could cure you of ills was an exercise in trial and error. So wouldn’t it be nice if nature (or God, who I guess would also be nature in a way) dropped hints as to which ones were good for the human body? Such thinking, known as the doctrine of signatures, actually developed with remarkable frequency all around the world from culture to culture. Plants meant to heal certain organs and body parts, like the liver or the eye, must show a certain “signature” by resembling the thing they treat.

So the bloodroot, with its red extract, was theorized to fix problems with blood. And the saxifrage, which breaks apart rocks as it grows, must relieve kidney stones. Venomous bites are covered too: Alkanet’s viper-shaped seeds help for snake bites, and the coiled shoots of the herb scorpius will take care of that scorpion sting lickety-split. Even using plants that grow in the same area where a disease like malaria is prevalent can be used as cures.

We now know that this is both wildly wrong and wildly dangerous. A sliced mushroom may look like an ear, but that doesn’t mean you should eat it to cure your earache (choose the wrong mushroom and you can add 24 hours of talking to furniture to your troubles). But as we shall see, the doctrine of signatures, when properly applied, has in fact been for some cultures an indispensable tool in medicine.

Long before the theory popped up in the West, peoples all over the world subscribed to what we now call the doctrine of signatures, from Asia to the New World. Native American tribes all used it, according to Bradley C. Bennett in his essay “Doctrine of Signatures: An Explanation of Medicinal Plant Discovery or Dissemination of Knowledge?” The Cherokee, for instance, thought the common purslane’s stalks, which resemble worms, could be used to treat worms in humans.

In the West the doctrine was first mentioned in the writings of Pliny the Elder, the brilliant Roman naturalist whose imagination nevertheless often outpaced his grounding in reality. In the 1500s, German-Swiss physician Paracelsus wrote at length on the topic, claiming that “the soul does not perceive the external or internal physical construction of herbs and roots, but it intuitively perceives at once their Signatum.”

During the 16th and 17th centuries, the doctrine of signatures was ubiquitous in the West. According to Bennett, the view was a decidedly theological one: God, in all his/her benevolence, shapes certain plants to resemble human organs as a clue, and we need to take the hint. And if you don’t take the hint, well, God will force it on you. In Milton’s Paradise Lost, first published in 1667, the Archangel Michael uses the eyebright flower to cure Adam’s eye infection.

And there was the British botanist William Coles, writing a decade before that epic poem: “Though Sin and Satan have plunged mankinde into an Ocean of Infirmities, yet the Mercy of God, which is over all his workes, maketh Grasse to grow upon the Mountaines and Herbes for the use of men, and hath not only stamped upon them a distinct forme but also hath given them particular Signatures whereby a man may read the use of them.”

This theological grounding is quite problematic, in the sense that it assumes the universe was created for humankind, then stocked with convenient medicines for the taking, the kind of anthropocentric worldview also manifested in the geocentrism that Copernicus overthrew in the 16th century. There’s also the rather glaring issue of subjectivity: That root may look like a kidney to you, but it sure looks a lot like a liver to me.

Not that he needed to disprove the theory, but in his essay Bennett presents the findings of his research into the efficacy of various plants with heart-shaped leaves in treating heart disease. Searching various databases, he found 2,584 plants with such a shape, and randomly selected 80. Of those 80, only 21 were used in medicine, and of those only three applied specifically to cardiac medicine. “These data clearly refute any a priori value of heart-shaped leaves as signs for cardiac activity,” he writes. Translation: Don’t go around eating morning glory leaves to try to cure your heart murmurs.

And even in the heyday of the doctrine of signatures there were plenty of detractors. The 16th century Flemish physician Rembert Dodoens called it “absolutely unworthy of acceptance.” Even Samuel Hahnemann, who founded the practice of homeopathy, vehemently attacked the doctrine, which would turn out to be just as epically wrong as his own theory. He said in 1825 with an irony so hilarious it’s almost incomprehensible: “I shall spare the ordinary medical school the humiliation of reminding it of the folly of those ancient physicians who, determining the medicinal power of crude drugs from their signature, that is, from their color and form, gave the testicle-shaped Orchitis-root in order to restore manly vigour…”

Hahnemann and Dodoens were right, of course. But there is the problem of these supposed cures that … uh … actually work. The doctrine of signatures, it turns out, isn’t totally worthless bunk. The Cherokee’s worm-like purslane, writes Bennett, is indeed “effective in controlling intestinal parasite loads and has gastroprotective activity.” And the Archangel Michael’s eyebright, he adds, can be loaded into eye drops to treat infections of the peepers.

This could be coincidence, sure. But more likely it’s the fact that the doctrine of signatures has at times not been used to identify cures, but to remember them, and in that way has been quite beneficial for peoples without a written language. Where the vast majority of scholars have roundly dismissed the doctrine as silly pseudoscience, Bennett sees the mnemonic benefits of the theory. That is, its use as a device to memorize what plants can repair what problems in the human body.

He cites another scholar, writing in 2002 about the medicine of the native Peruvian peoples: “In these and other orally transmitted systems of thought, mnemonic cues may be essential to the viability of knowledge transmission. Plants that are both efficacious and easy to remember are more likely to be maintained in the pharmacopoeia of non-literate societies through time.”

In Europe, too, the doctrine of signatures was often applied after the plant’s efficacy had already been established. Bennett relates, for instance, the story of the discovery of willow bark’s powers. A reverend by the name of Edward Stone had accidentally tasted it, and found that its bitterness was much like that of cinchona bark, used to treat malaria. He then discovered that the willow could also be used to treat fevers, malarial or otherwise. And this, he concluded, is because the willow “delights in a moist and wet soil,” an environment where malaria is common. Cures, he reasoned, must occur near their causes.

The doctrine of signatures was of course totally undone by a dash of common sense and more rigorous experimental standards as science evolved. So we certainly, certainly, don’t have any business propagating the theory in 2014. Yet it’s happening. Leading people to believe that they can treat depression by eating walnuts because they look like brains is dangerous and irresponsible, just like saying homeopathy can treat, oh I don’t know, anything at all. These are dead theories, and their rotting corpses must be buried to keep them from stinking up the place.

So if you’ll excuse me, I’m off to chew on some ivy leaves to treat this elevated blood pressure of mine.


Bennett, B. (2007) Doctrine of Signatures: An Explanation of Medicinal Plant Discovery or Dissemination of Knowledge? Economic Botany. Vol. 61, No. 3, pp. 246-255