Law of similars

The law of similars or similarity principle (Samuel Hahnemann: similia similibus curentur) is one of the two basic concepts of homoeopathy. According to this principle, every disease is said to be curable by substances causing similar symptoms when taken by a healthy patient. The law of similars is not only a rule of thumb in homoeopathy but is understood as a basic law of nature.

The law of similars was not invented by Hahnemann, just rediscovered and made popular. It has been handed down from Hippokrates: "By similar things a disease is produced and through the application of the like is cured." The sentence Contraria contrariis curantur is also attributed to Hippocrates and Galenus. Paracelsus said something similar: "similia similibus curantur". It has to be noted here that the indicative (curantur - cured) is different from the subjunctive mood used by Hahnemann (curentur - shall be cured). Hahnemann distanced himself strictly from the Doctrine of Signatures.

In academic medicine it is applied in the broad sense for allergen immunotherapy and for vaccinations. In general there is the problem, that for the term "similar" no scientific definition exists. Similarities are not an objective feature, but created by pattern recognition in the brain and depends on the knowledge of the observer and his power of observation. It depends furthermore on type and quality of the inspection.

Taken serious scientifically, the principle is easily disproved. It does not take more than to come up with one disease which cannot be cured by similars. One such example is sufficient to falsify (disprove) a theory expressed so broadly. Even viewed as a mere rule of thumb, it may be expected to find a vast majority of diseases which will be cured by remedies causing similar symptoms. However, such a general proof of efficacy could not be provided. Therefore, the law of similars corresponds to magical thinking and has no relation to science.

Samuel Hahnemann, the founder of homoeopathy, referred to his successful chinchona test which had inspired the law of similars. This experiment, however, could not be replicated, and is to be seen as one of the errors of homoeopathic doctrine. The law of similars already had been discussed prior to Hahnemann, but was made popular first through homoeopathy.

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• Deutsch: Simile-Prinzip