Homeopathy is a medical theory developed at the beginning of the XIX century by Samuel Hahnemann. Hahnemann was a highly cutivated and acute physician and his theory contains traces of the most influential medical theories of ancient Greece, dating back to Hippocrates (V century b.C.), most notably the hypothesis that a disease can be cured by a drug causing similar symptoms. Unfortunately Hahnemann was extremely critical towards contemporary physicians and theis theories (at the time already advanced), denied the importance of pathology and based his entire medical system on the analysis of the symptoms, an obsolete position. Moreover, as Hahnemann grew older, he leaned towards a dogmatic mysticism, and criticized not only the non homeopathic physicians, but also those of his followers who did not accept entirely and without questions his theories, that included "spiritual" entities to explain diseases, physiology and pharmacology. After Hahnemann's death hs followers founded different schools, some more moderate and prone to compromise with conventional medicine (the so-called "half-homeopathy"), other more extreme. The best known logical and empirical defect of homeopathy is the use of so extreme diluitions, that the homeopathic "remedy" is completely devoid of the active substance after which it is named. The efficacy of homeopathic therapies, is modest at best and does not support the pretences of modern homeopaths; moreover the studies reporting the best results are often those with faultier methodology. Important damages and collateral effects due to homeopathic therapies have been reported, most often caused by the accidental or intentional adulteration of the remedies.

Other web resources on homeopathy:
CICAP Homeopathy
Quackwatch on homeopathy
NCAHF on homeopathy
A look to the other's view point: some websites of homeopathic societies:
Homeopathy Home (this web sites offers several relevant informations, including a full online copy of Hahnemann's Organon)
The Italian Society of Homeopathic Medicine (SIMO)


The theory of homeopatic medicine was developed by Samuel Hahnemann (1755-1843) since 1796, and was modified and evolved by his followers in different ways, often reciprocally incompatible and incompatible with the original. Its fundamental tenets are:

1) the law of the similes, which states that a disease can be cured by administering a drug (or a poison) causing similar symptoms: similia similibus curentur (let like be cured by like) or similia similibus curantur (like is cured by like).

2) The hypothesis of vitalism, stating that the living organisms possess a specific kind of energy, the dynamis or vital force. Hahnemann also thought that the vital force had a spiritual nature, but this was not shared by all scientists who believed in vitalism.

3) The law of infinitesimals, which implies that a homeopatic drug (or remedy) remains active, or indeed becomes more active, if the dose is lowered, or even annihilated, by extensive serial diluitions.

Like all revolutionaries, Hahnemann refused and criticized the medical tradition and encouraged his followers to forget the classics and to experiment. However, he also wrote several books, thus he wanted that medical classics be abandoned, not books in general and his own in particular. Hahnemann did not conceive, even as a possibility, that the classics of medical literature could contain genuine empirical information, as good as his own. The absurdity of Hahnemann's pretences is most evident if one considers the case of anatomy, pathology and morbid anatomy: Hahnemann and his followers never practiced autopsies, thus the refusal of medical classics implied that homeopaths remained completely ignorant about these issues. Indeed it is impossible to fully investigate even the smallest and most circumscribed scientific problem in one's lifetime, without recurring to the observations of others. A century later, some of Hahnemann's followers pretended to denounce the momentous discoveries of Koch and Pasteur, thus strenghtening the isolation of homeopathy from mainstream medicine, and making it more conspicuous for what it lacks than for what it contains.

The LAW OF THE SIMILES (or the similars) is part of an ancient medical tradition [1], that Hahnemann believed to have rediscovered after an experiment he carried out on himself, by taking extracts or powder of the bark of Cinchona, the standard drug for malaria (Cinchona is the source of quinine, a drug still used to treat malaria). Hahnemann thought that the symptoms he suffered because of Cinchona intoxication were similar to an attack of malaria and made the hypothesis that the efficacy of the drug were due to its ability to cause symptoms similar to the ones it cured: the law of the similars. This experiment (carried out in 1790) is very relevant for the development of the theory, and deserves a detailed discussion [2].

The law of the similars, or similia principle, appears in a book published in 1796, and states that a disease can be definitively cured by a drug that causes in the healthy man symptoms similar to those that afflict the sick. Hahnemann thought that diseases are dynamic, spirit-like, alterations of the vital force, and that symptoms are their only material counterpart. In this theory, symptoms are not "caused" by the disease: they manifest the alteration of the vital force. This idea is better understood if we consider that Hahnemann believes sensation and perception to be among the functions of the vital force; thus abnormal perception (e.g. pain) is one and the same event as the perturbation of the vital force [3: Hahnemann, Organon VI ed. 71], and no space is left for pathology. Pathology was an established science at the time [4], and Hahnemann's theory was born obsolete: indeed autopsies had already demonstrated that very often subjective symptoms can be traced to anatomical lesions.

Hahnemann thought that only a single drug should be administered at a time, and that a second dose (or a different drug) should be given only when the effects of the first are completely over (but he later suggested that the dose should be repeated every day), and the patient has not been cured. Unless the first drug is incorrectly chosen (i.e. it is unhomeopatic to the disease and causes symptoms different from those of the patient), it will remove at least part of the symptoms, and the disease must be completely re-evaluated before deciding the next drug.

Hahnemann provides two distinct, not mutually exclusive, explanations of the law of the similes:
1) the HYPOTHESIS OF THE CURATIVE DISEASE: since diseases have no material basis (i.e. no pathology, no microbiology, etc.) it is possible to replace the natural disease of the patient with a similar one caused by the drug (i.e. iatrogenic). At variance from the natural disease, the iatrogenic one is controlled by the physician who can stop its course by discontinuing the drug. Cure is therefore mediated by the replacement of a disease with another. There are two requirements for this strategy to succeed, i.e. that the iatrogenic disease is stronger than the natural one, and that the symptoms of the two diseases are very similar [
5: Hahnemann, Organon VI ed. 26]. This hypothesis is part of the history of medicine, even though in an empirical, rather than systematic, form.
2) The HYPOTHESIS OF THE REACTION OF THE ORGANISM: the homeopatic drug stimulates a reaction of the organism stronger and more effective than that induced by the disease and stimulates reparative processes [
6: Hahnemann, Organon VI ed. 66]. Since the stimulus is similar to that of the natural disease, as guaranteed by the identity of the symptoms, the reparative processes induced by the homeopatic drug go in the right direction. Also in this case parallels may be found in the most ancient greek medical writings.

The two hypotheses are neither alternative, nor logically correlated: they are distinct and independent to each other. Indeed, one could surmise that the organism reacts more strongly to the therapeutical disease than to the natural one, but this third hypothesis is not implicit in either or both the original ones. Unfortunately both of Hahnemann hypotheses are false and do not correspond to any fact, if not under special circumstances that do not allow any generalization. Indeed it is very common that two different or similar diseases coexist in the same patient, neither of them curing the other, and the reactions of the organism may or may not result in a cure, depending on the disease considered. As a consequence, the analysis of Hahnemann's hypotheses is purely logical, no empirical evidence can be advanced in their support and they belong to the history of medicine, not to modern medical science. If homeopathic therapies are effective (and there are excellent reasons to doubt) this is because of reasons other than those propounded by Hahnemann.

The biological theory of VITALISM had been formulated by G. Stahl (a chemist) in the XVII century and was quite popular during Hahnemann lifetime (and later). Vitalism counts as its followers such famous scientists as Bichat and Pasteur. Homeopathy is not the only medical theory based on vitalism, and Hahnemann was a late convert: it seems that he adopted vitalism in 1820 or later, to explain the law of the similes he had published in 1796; indeed he modified vitalism and built up a new and complex theory whose tenets were not adopted by the vitalists of the time (e.g. by Bichat).

The fundamental hypothesis of vitalism postulates that living beings are endowed with a special and characteristic energy, the vital (life) force, whose alteration or loss causes their death. Stahl and Bichat thought that the organism were the theatre of a continuous war between chemical and physical phenomena, whose end was the death of the organism, and the life force, that kept the organism alive [7]. These authors did not know that putrefaction and decomposition of the corpse are due to bacteria: this discovery is due to Pasteur and follows his works on fermentation published in the 1870s. It was reasonable for Stahl and Bichat to assume that decomposition and decay were caused by the chemical forces, not antagonized by the life force; life force was a natural energy, much like any physical energy, that required scentific investigation.

Hahnemann's idea was different: he thought that the vital force were a spiritual entity [8]. Hahnemann adopted vitalism relatively late, much after his experiment with Cinchona and the law of the similes: he saw vitalism as a theory capable of explaining his unreliable discoveries. Hahnemann thought that diseases were alterations of the vital force, and that their symptoms were the observable counterpart of the alteration of the vital force. In this hypothesis the symptom is not caused by the disease, it is the disease itself, or, at least, the material and observable part of the disease. This logical shortcut is made possible, in Hahnemann's mind, by the assumptions that the vital force is responsible for sensation and that symptoms are anomalous sensations; thus the symptom is not the clue that leads to the diagnosis, it is our direct perception of the disease. Galen had compared the physician interpreting the symptoms to establish the diagnosis to the hunter who follows the footprints of his prey, but to Hahnemann the symptoms corresponded the direct sight of the prey. Drugs possess, according to Hahnemann, a vital force in their own, capable of perturbing the vital force of the patient, and cure is made possible by this interaction between spiritual energies, and the consequent iatrogenic (i.e. medically induced) disease.

In spite of Hahnemann's best attempts, vitalism does not satisfactorily explain the law of the similes. Indeed an obvious defect of the hypothesis is that there is no reason why the terapeutical disease substitutes for, instead of coexisting with, the natural one (so that the homeopathically treated patient gets two diseases instead of one). To avoid this objection, Hahnemann postulated that the vital force has no memory and cannot "remember" the natural disease in the presence of the terapeutical one [Organon, V ed., par.34]. But if this addition to vitalism were accepted, it would imply that non lethal diseases can be maintained only if their cause persists, otherwise they would be forgotten by the vital force also in the absence of the homeopathic therapy. Thus, long lasting diseases cannot be, as Hahnemann pretends, of purely spiritual nature and the theory is self-contradictory. On the other hand, if the cause persists, then the homeopathic therapy cannot succeed because after the action of the remedy is over the natural disease would be reestablished.

The LAW OF THE INFINITESIMALS (or of the small doses) postulates that the effect of the drug is increased, rather than decreased, if the drug is diluted (and therefore the dose administered to the patient is reduced), provided that dilution is accompanied by a vigorous shaking (succussion).
Hahnemann's drugs were poisons almost by definition, because to obey the law of the similes they had to be able to induce symptoms analogous to those of the patient's disease. As a consequence, administering homeopathic drugs implied a significant risk, as Hahnemann himself had discovered in his experiment with Cinchona bark. To diminish the risks and the distress of his patients, Hahnemann began to dilute his drugs (or "remedies"), that at the time were mostly vegetable extracts or mineral powders.

Hahnemann's method was as follows: the desired drug is dissolved in water or in a mixture of alcohol and water at a concentration of 1 part in one hundred (if the original substance is a mineral powder this is achieved by dissolving 1 g in 100 ml; if it is a liquid vegetable extract the initial dilution is 2 ml to 100 ml to account for the extracting solvent) the first dilution so obtained is called the "mother tincture" and has the nominal titer 1c to indicate the first centesimal dilution. If the substance is insoluble, 1 g is thoroughly mixed (triturated) with 99 g lactose and the 1c is a powder instead of a solution. 1 ml of the 1c is then diluted to 100 ml with fresh solvent and succussed to obtain the 2c; in the case of lactose powders the 2c is obtained by trituration of 1g of the 1c with 99 g of lactose. The whole procedure is repeated over and over again to obtain the 3c, 4c and so on up to the 30c and over; from the 4c on the powders become soluble and can be diluted and succussed in water, instead of lactose. The centesimal series here described is not the only one that is used (the decimal or X series and the 50,000th or XL series are also used), but is the most common. Since the number of molecules of the drug present in the mother tincture can be calculated (even though it is not the same for all drugs, depending on their molecular weight and their purity) it is unequivocal that at dilutions equal to or higher than 11-12c, and often even lower, the dose administered to the patient does not contain a single molecule of the drug. It is also pertinent that homeopathic remedies of vegetable or animal origin are crude mixtures of many different substances, each already relatively diluted in the mother tincture.

Homeopaths often refer to their dilutions as "potencies", and to their preparation as "potentization" or "dynamization" to mean that the drugs so treated are more effective than in the extracts. They use low (4c-8c) as well as medium (15c-30c) and high potencies (>30c). High potency homeopathic remedies do not contain any single molecule of the substance after which they are named, and this has always been a formidable difficulty to the acceptation of homeopathy by physicians and scientists, but it is important to point out that low potencies do contain the intended drug, albeit quite diluted.

Hahnemann's ideas about dilution underwent an evolution during his lifetime, that has been reconstructed by the british homeopath R. Hughes. In 1796-1798 Hahnemann administered his remedies in doses of a few grains (1 grain = 65 mg), whereas in 1799 he had begun to dilute the same drugs in mixtures of water and ethanol to 1:2.000 and typically prescribed 10 drops of the solution thus obtained. In his works published in 1801-1806 Hahnemann describes diluitions up to 1:1.000.000 (that corresponds to the 3c). Only after 1809 Hahnemann's method became standardized on the serial dilutions by 10- or 100-fold, still in use today [9: Hughes, 1878]. In parallel with the increase of dilution, Hahnemann modified also the law of the infinitesimals: he had begun to dilute his remedies in order to make them less toxic and more acceptable to his patients, but later on he became convinced that dilution increased the efficacy, whence the concept of "potency". Indeed when Hahnemann adopted the hypothesis of vitalism, around 1821, he attributed the effect of his drugs to their (spiritual) vital force, that the processes of dilution and succussion could release from their material gaol. This hypothesis was absurd even with respect to its vitalistic premises since the drugs were extracts of dead vegetable specimens or mineral substances, by definition devoid of vital force.

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Cure by similes (i.e. by means of drugs, poisons or physical stimuli that induce symptoms similar to those of the patient), even though apparently counterintuitive, belongs to the hystory of medicine and was commonly employed two millennia before Hahnemann. Indeed it is described in the Corpus Hippocraticum, a heterogeneous collection of writings dating back since the V century b.C., that Hahnemann knew because of his medical studies and his work as a translator. Cure by similes was not fashionable in the XVIII century (it had been somewhat dismissed by Galen and his followers since roman times), but it was resurrected by Hahnemann after an experiment on Cinchona bark, an antimalarial.
The Corpus Hippocraticum includes writings by authors who favor the therapy by similes as well as writings that recommend the use of contraries. This discrepancy is due to the fact that the Corpus collects writings belonging to at lest two opposite schools of medicine, whose hypotheses on the nature of disease are incompaible with each other. The Kos school maintains that the disease is a natural process, representing the reaction of the whole body to some noxious external stimulus, whereas the authors belonging to the Knidos schoold interpret the disease as the altered functionality of some organ. The former theory is more linked to empirical, but imprecise, observations, the latter is more inclined to theoretical speculations; both, however, are based on the concept of the body humours (fluids).
The THEORY OF THE HUMOURS did never reach a logical maturity and is therefore quite difficult to formalize; it postulates that the fundamental physiological process is the production of some body liquids. The humour whose fate is described more thoroughly is the blood, thus to get an idea about the theory, it is convenient to follow the (hypothesized) formation and use of the blood. The description I report here is however a late one, and follows Galen's reformulation of Aristoteles' hypthesis. The BLOOD is initially formed in the liver out of the alimentary substances absorbed by the gut and transported via the mesenteric veins. The maturation of these substances is called CONCOCTION, and is supposed to be similar to food cooking. The newly formed blood leaves the liver through the hepatic veins and reaches the heart via the inferior vena cava. In the heart the concoction continues and blood is mixed with other humours and with the pneuma (the breath). The mature blood thus formed leaves the heart through the arteries and veins and reaches all tissues and organs of the body, were it is consumed or transformed into solid tissue. Thus the blood circulates only once from the liver to the tissues, as water is used to irrigate farming fields. The other humours, three in number according to some authors (phlegm, yellow bile and black bile), more according to others, share a similar fate, but follow less defined pathways and are concocted in different organs (e.g. the brain would be relevant for the maturarion of the phlegm and the spleen for the black bile). The theory attributes to the organs two functions: they are places where specific humours are concocted and mixed (this being a "vegetative" function) or they are necessary to sensory, motor, rational and emotional processes, and consume the humours; some organs are relevant for both types of functions (e.g. the heart).
HIPPOCRATES OF KOS: THE DISEASE AS A PHYSIOLOGICAL PROCESS AND THE LAW OF THE SIMILES. In the most ancient writings of the Corpus Hippocraticum, attributed to the medical school of Kos, the disease is described as a condition in which the concoction of humours is altered or inhibited. Some poorly defined factor (e.g. cold, wind, inadequate nutrition, etc.) causes a crude, unconcocted humour to accumulate; the organism reacts by increasing the concoction processess. When the physician is called for, the cause has already acted and the disease reflects the attempts of the body to restore the balance of its fluids.
The physicians adopting this theory were aware that their art had not the power to concoct the humours inside the patient's body, and their objective was to help the reparative processes that would in any case occur. Fever is the paradigma of a reparative process: the organism raises its temperature to concoct the crude humour; the physician warms up his feverish patient with blankets and warm baths to further increase the concoction. Healing occurs either because the humour is finally matured or, if in excess, is expelled (as vomit, sweat, pus, etc.). This theory provides the rationale of some famous aphorisms: the role of the vis medicatrix naturae in the healing process (the organism possesses stronger curative mechanisms than the physician) and the primum non nocere (since the principal cause of the cure is the diseased organism itself, the physician must avoid any intervention that could cause damages or hinder the physiological processes, and provide a bland support).
In this theory the disease is first and foremost the auto-curative reaction of the organism, and also the anatomical lesion (i.e. the accumulation of an unconcocted humour) is of internal origin. Symptoms are exagerated physiological processes, that are also present, to a lesser intensity in the healthy state of the organism; coherently, therapy can only be through SIMILARS. Symptoms like sweating, vomiting, diarrhoea and emission of pus are viewed as attempts of the body to excrete crude and unconcocted humours produced in excess, and it is likely that observation of these fluids contributed to shape the whole theory. The view here presented had some implications, fully recognized by the authors of the Corpus Hippocraticum: i.e. that diagnosis is poorly relevant (the physician must help whatever process is occurring in his patient), and that all symptoms are relevant, especially those characteristic of each single patient (the "propria"). Since each patient has his/her own disease and his/her own ways to react, therapy is to be individualized.
It is likely that the therapy by similes has a second logical root. The authors of the Corpus Hippocraticum thought that healing can occur via either of two alternative mechanisms: the slow and progressive lysis and the abrupt crisis, usually ushered in by an apparent aggravation of the symptoms. Thus, therapy by similes might have been viewed as an attempt to induce the crisis (whereas the contraries might oppose the disease and promote the lysis). The crisis was by no means a "safe" event, and it could cause the death as well as the recovery; however the mode of healing, by lysis or by crisis, was thought to be characteristic of the disease, and a disease known to heal by crisis could not be forced to heal by lysis.
THE KNIDIAN SCHOOL: THE DISEASE AS A NON-PHYSIOLOGICAL PROCESS AND THE LAW OF THE CONTRARIES The authors of the school of Knidos, whose writings belong nonetheless to the Corpus Hippocraticum, propose a theory different from the one described above, whose main point is that disease is not a physiological process, however distorted or exaggerated, but something extraneous to the physiology of the organism, to be opposed and if possible extirpated. The pathological humour (pus, vomiting, etc.) is not crude or unconcocted, but has matured in a wrong and damaging way, and should never be present in the healthy organism; its concoction is possile only under exceptional circumstances and as a general rule healing requires that it is removed, if necessary by surgery, and its further production is blocked. This theory justifies therapy by CONTRARIES, reduces the relevance of the vis medicatrix naturae (since if it were active the pathological processes should be inhibited) and attributes to the physician a more active role, up to the point of rejecting the primum non nocere: it is better to accept the risk of a dangerous therapy than to let the disease to progress undisturbed, since there is no warrant that healing will ensue.
The implications of the hypothesis of the Knidians are almost opposite to those of the school of Kos: i.e. the diagnosis is of utmost relevance, and the symptoms that are common amongst different patients (the "communia") are the most informative. Indeed, since the symptoms are not viewed as attempts of the body to heal, but as pathological processes, it becomes imperative to decide how harmful they are and whether or not they have to be counteracted and how. This can only be accomplished by classifying the diseases and diagnosing each patient; and once the diagnosis becomes an issue, the communia become much more relevant than the propria. Individualization is not any more searched for, since it stands against diagnosis.
There are obvious defects of observation and deduction in both theories of disease, justified only in part by the inadequacies of the methods of the time. A gross conceptual error is that diseases are viewed as essentially homogeneous, and one single theory can encompass all of them. Indeed some diseases may be better described as alteration of physiological processes, and their symptoms predominantly express the reactions of the organism; other diseases may be dominated by pathological processes, devoid of a physiological counterpart, and cause mostly symptoms that cannot be described as reactive. Perhaps the majority of disease has both reactive and non-reactive components. Transmissible diseases were known since the V century b.C. (e.g. the plague of Athens, as described by Tucidides) and coexisted with non-transmissible ones; the former correspond to the hypothesis of disease as extraneous to the physiology of the organism much more closely than the latter.
Hahnemann, as a scrupulous physician, distinguished PRIMARY SYMPTOMS, caused by the perturbation of the vital force and thus extraneous to the physiological processes of the healthy organism and SECONDARY SYMPTOMS, caused by the reaction of the vital force against the perturbation and thus belonging to the repertoire of physiological processes; in this sense he tried to improve the classical medical tradition.
THERAPEUTICAL DISEASES. The writings of the Corpus Hippocraticum include, every now and then, the observation that a disease can occasionally heal if the patient is stricken by a second disease; this hypotesis is very relevant to Hahnemann's theory. It is not obvious from the descriptions found in the most ancient medical writings whether or not the authors intend to propose the hypothesis of a true therapeutical disease, since they do not clearly distinguish different diseases and the crude humour may settle in any organ of the body and may probably also move from one to another. Thus, what seems to be considered as a therapeutical disease may have been intended instead as different localization of the original disease. The therapeutical disease is not as common in the Corpus Hippocraticum, and by no means is taken as a typical step towards healing; indeed in reality it is very uncommon that one disease cures another, and the observation of two independent diseases occurring at the same time in the same idividual must have occurred quite often.
THE THEORIES DEVELOPED BY PARACELSUS (THEOPHRASTUS HOHENHEIM). Theophrastus Hohenheim (1493-1541), had changed his name to (or adopted the nickname of) Paracelsus to match his achievements with those of Celsus, a famed roman physician. He was the author of a theory of the similes not unlike the greek one and must be considered Hahnemann's precursor, even though the latter always denied having read Paracelsus' writings (and this is hard to believe). The biographies of Paracelsus and Hahnemann closey resemble each other: both men strongly criticized the medical establishment of the time, getting involved in quarrels with the local physicians and apothecaries; as a result both of them moved through several german speaking cities; both of them were interested in chemistry and adopted for their clinical use several new substances, notably inorganic salts. Moreover both Hahnemann and Paracelsus defended old theories preteding that they were new, a fact that must have been inacceptable to their physician fellows, and both of them defended their original observations against the tradition, neglecting that the tradition was based on equally good, or perhaps better, observations. Paracelsus came to the point of publicly burning some respected texts of medicine. Neither Hahnemann nor Paracelsus refrained from accusing the contemporary physicians of consciously or unsconsciously killing their patients. Paracelsus thought that a similarity existed between the disease and its cure, but he did not limit this resemblance to the symptoms (as Hahnemann would do three centuries later), and some of his prescriptions were quite extravagant.

Hahnemann's experiment with Cinchona bark at very high dosage, carried out in 1790, played a pivotal role in the development of the theory of homeopathy, and stands out as unique in the history of the discipline: no similar experiment has been carried out since. The logic of the experiment is simple: Hahnemann knew that Cinchona bark cures malaria, but did not know how and why, and was dissatisfied with the explanations proposed by contemporary physicians. Moreover, Hahnemann, as any physician of his time, was familiar with the symptoms of malaria, quite a common disease at the time (though probably not as common in Germany as in warmer countries) and could diagnose the disease with great accuracy.
Hahnemann and his contemporaries did not know the cause of malaria: the Plasmodium parasite that causes the disease was identified by Laveran in 1884, and its life cycle, in which man plays the role of the definitive host, was to be discovered by Ronald Ross in 1897 and, independently, by the Italian malariologists Grassi, Bignami and Marchiafava in 1898-99.
Hahnemann hypothesized that Cinchona bark had effects on the organism, even in the absence of malaria, and that its therapeutic effectiveness was to be attributed to these effects. Subsequent research (not by homeopaths) demonstrated that Hahnemann's hypothesis was wrong: quinine, the active ingredient of Cinchona bark, acts by killing the Plasmodium, even outside the human body; thus, the effects of the drug in man, and in particular in the healthy man, are irrelevant to its therapeutic effects.
Hahnemann, without knowing that his hypothesis was flawed, went on with his experiment, and ingested very substantial amounts of Cinchona till he suffered of the grave intoxication that we name cinchonism. The symptoms of cinchonism do not resemble those of malaria, and these two pathological conditions are quite different; however to Hahnemann cinchonism seemed similar to malaria and he rationalized this presumed similarity as a causal relationship: the bark cures malaria because it induces similar symptoms. The undue and arbitrary generalization of this unique experiment, erroneous in its premises and in its conclusions is the law of the similes, founding stone of homeopathy.
The law of the similes had a point in its favour: it was not a novelty, and cure by similes had been practiced and advocated since the origins of greek medicine, in the writings of the Corpus Hippocraticum, a collection of texts dating back to the V and the IV century b.C.; a more recent convert to this hypothesis had been Paracelsus (Theophrastus Hohenheim, 1493-1541). It seems that Hahnemann denied ever reading Paracelsus, and rarely, if ever, names him and his theory in the Organon. To Hahnemann contemporaries, the law of the similes was, at best the re-edition of a classical theory; at worst the attempt to resurrect an obsolete medical folklore. In any case it was not, and did not look like, a therapeutical breakthrough as modern homeopaths pretend, implying that Hahnemann was ahead of his time.
If we neglect the superficial plausibility warranted by conformity to older theories, Hahnemann's interpretation of the Cinchona experiment is not stringent and has been criticized also by some homeopaths, notably by the british Richard Hughes. Hughes remarked a grave logical error in Hahnemann's interpretation of his experiment: malaria can present typical symptoms like the recurrent fever, tertian or quartan, or with less typical symptoms like pernicious fever, blackwater fever, and cerebral malaria. Cinchona cures typical as well as rare forms of malaria, in spite of the wide differences between their symptoms; by contrast Cinchona does not cure the relapsing fevers of non malarial origin, in spite of the similarity of their symptoms to those of typical malaria. Hughes conclusion is that Cinchona is specific for malaria, rather than for its symptoms, and implicitly denies or modifies the law of the similes that in its original formulation deals with symptoms. Indeed Hughes suggests that Cinchona causes an intoxication globally (rather than just symptomatically) similar to malaria, and different from that of non malarial syndromes, thus invoking those pathological factors that Hahnemann had refused (Hughes 1893, A Manual of Pharmacodynamics, ed. B. Jain, New Delhi, India, 2001; lecture XXVI, p. 399).
After the Cinchona experiment, Hahnemann was so convinced of the law of the similes that he did not try to repeat it using other drugs of proven efficacy, and tested instead the toxic effects of several substances, mostly vegetable extacts, that were often known poisons, in order to increase the number of drugs available to the homeopathic physicians. At the time some drugs of proven efficacy against specific diseases were known, on which Hahnemann could have tested his hypotheses: an important example was lemon juice, the elective therapy and prevention of scurvy. The discovery was due to the scottish physician James Lind who served in the british navy on board of HMS Salisbury. He tried several substances on the saylors suffering from scurvy, from 1747 on, and published the discovery that lemon juice was effective in his "Treatise on the scurvy" (published in 1753). It would have been easy for Hahnemann to test lemon juice on himself or on his followers, to see whether it causes symptoms similar to those of scurvy, namely hemorrages and loss of the teeth; but he did not. One could think that the experiment would have been irrelevant to homeopathy for, if lemon juice were able to cure a disease whose symptoms it cannot reproduce, this would only demonstrate that there are exceptions to the law of the similes, not that the law is false. This objection is correct in theory, but does not capture Hahnemann's ideas: the man had a romantic view of medical science and thought that the only possible reason of the efficacy of a drug was to be homeopathic with the disease (i.e. capable of inducing similar symptoms); no exception was admissible:
"This depends on the following homoeopathic law of nature which was sometimes, indeed, vaguely surmised but not hitherto fully recognized, and to which is due every real cure that has ever taken place: a weaker dynamic affection is permanently extinguished in the living organism by a stronger one, if the latter (whilst differing in kind) is very similar to the former in its manifestations." (S. Hahnemann, Organon, VI ed., 26, italic added)

3: "... human diseases are nothing but groups of certain symptoms ...". Hahnemann (1843), Organon VI ed. 71, english translation by J. Kunzli et al., Orion Books ltd., Londra, 2003. On the same subject se also ibid. 14-17.

4: It is commonly assumed that the manifesto of modern pathology and morbid anatomy is G.B. Morgagni's "De sedibus et causis morborum per anatomen indagatis" (published in 1761, when Hahnemann was six years old, and immediately translated in english, french and german). However, in his book Morgagni quotes other texts (e.g. Bonnet's Sepulchretum anatomicum) and the experience of other anatomists (e.g. his master Valsalva), and effectively makes clear that a solid tradition of morbid anatomy and pathology was already established.
Pathology was a bete noire for Hahnemann: "If the patient succumbs, the practiser of such a treatment is in the habit of pointing out to the sorrowing relatives, at the post-mortem examination, these internal organic disfigurements, which are due to his pseudo-art, but which he artfully maintains to be the original incurable disease. Those deceitful records, the illustrated works on pathological anatomy, exhibit the products of such lamentable bungling." (Organon V ed., par.74, footnote).

5: "In the living organism a weaker dynamic affection is permanently estinguished by a stronger one, which, though different in nature, nevertheless greatly resembles it in expression." Hahnemann (1843), Organon VI ed. 26, english translation by J. Kunzli et al., Orion Books ltd., Londra, 2003. On the same subkect see also ibid. 34.

6: "The primary action that some of these remedies produce is perceptible to a sufficiently attentive observer, but the counteraction (secondary action) of the living organism is only as much as is needed to restore the normal condition." Hahnemann (1843), Organon VI ed. 66, english translation by J. Kunzli et al., Orion Books ltd., Londra, 2003.

7: "La vie est l'ensemble des fonctions qui resistent a la mort." (Life is the group of functions that oppose death.) Bichat (1800) Recherches Physiologiques sur la vie et la mort. Ed. Flammarion, Parigi, p.57.

8: "In the state of health the spirit-like vital force (dynamis) animating the material human organism reigns in supreme sovereignity". Hahnemann (1843), Organon VI ed. 9.
"Without the vital force the material organism is unable to feel, or act or maintain itself. Only because of the immaterial being (vital principle, vital force) that animates it in health and in disease can it feel and maintain its vital functions." ibid. 10.

9: R. Hughes, British Journal of Homoeopathy, Aprile 1878. Reprinted as an appendix to R. Hughes, A manual of pharmacodynamics VI ed. (1893), B. Jain Ltd. New Delhi, India, 2001.

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