Homeopathy rests on a principle known as the 'law of similars'. Hahnemann first expressed it as the exhortation similia similibus curentur or "let likes cure likes." The idea did not originate with Hahnemann, but he was the first to use it as the basis of a system of medicine. The law of similars is generally considered by homeopaths as a law of nature. Following this principle, the appropriate homeopathic substance for treating a disease is one which induces similar symptoms in a healthy person.
The relation of similarity is primarily determined through provings, in which relatively healthy volunteers who are given a substance in homeopathic form record changes in their physical, mental, emotional, and spiritual symptoms. This information is subsequenly compiled and presented remedy-by-remedy in a Materia Medica. Subsequent versions of the Materia Medica additionally incorporate symptoms observed to have been cured by the remedy. A homeopathic repertory is an index of the Materia Medica, namely a listing of symptoms, followed by remedies reputed to cure them. With the growth of information on remedies such an index has become an indispensable tool for narrowing down the range of possibilities of appropriate remedy for a given case, although it is still properly treated as an adjunct to the Materia Medica.
At first, Hahnemann proved substances known to him as poisons or as remedies. Hahnemann's findings from provings were first recorded in his Materia Medica Pura.
's Lectures on Homoeopathic Materia Medica (1905) lists 217 remedies, and modern drugs and chemicals are being added continually to contemporary versions. As a result, homeopathy uses a wide variety of animal, plant, mineral, and chemical substances of natural or synthetic origin. Examples include Natrum muriaticum (sodium chloride or table salt),lachesis muta (the venom of the bushmaster snake), Opium, and Thyroidinum (thyroid hormone). Other homeopaths during and after Hahnemann's time, notably Hering and Lux, developed remedies called nosodes, which are homeopathic dilutions of the agent or the product of the disease in question. Rabies nosode, for example, is made by potentizing the saliva of a rabid dog. Some homeopaths also use a number of more esoteric substances, known as imponderables because the prepartions do not originate from a material substance but from electromagnetic or electrical energy presumed to have been captured by direct exposure (X-ray, Sol (sunlight), Positronium, and Electricitas (electricity)) or through the use of a telescope (Polaris). Recent ventures by individual homeopaths into the realm of esoteric substances include Tempesta (thunderstorm), and Kent wall. Berlin
Today, about 3000 remedies are used in homeopathy, of which approximately 300 are used based on comprehensive Materia Medica information, a further 1500-or-so on relatively fragmentary knowledge, and the rest are used experimentally in difficult clinical situations based on the law of similars, either without empirical knowledge of their homeopathic properties or through purely empirical knowledge independent of the law of similars. Examples include: the use of an isopathic (disease causing) agent as a first prescription in a 'stuck' case, when the beginning of disease can be traced to a specific event such as vaccination; the use of a biologically or chemically related substance when a remedy fails to act yet seems well-indicated; and more recently, the use of substances based on their place in the natural classification of their respective kingdoms (the periodic table or relevant biological taxonomy). This last approach is considered very promising by progressives in the homeopathic community, because it allows for grouping remedies and classifying the ever-burgeoning Materia Medica, but is rejected by many purists because it involves speculation about remedy action in the absence or proper provings.
The law of similars is the guiding principle in homeopathy, but calling it a "law" is misleading. It is, rather, an axiom or postulate which forms the foundation of the homeopathic system and through the application of which homeopaths arrive at their diagnosis. As it cannot be disproved scientifically (since a failure to cure homeopathically can always be attributed to incorrect selection of a remedy), the law of similars derives its justification from its contribution to the clinical results that homeopaths claim. The application of this principle, however, is not straightforward: there exist multiple methodologies for determining the most-similar remedy (the simillimum), and homeopaths will often disagree on the diagnosis. This is due in part to the complexity of the idea of 'totality of symptoms' to which the law refers, as homeopaths will not use all the symptoms of a patient, but will decide which are the most characteristic; this evaluation of the gathered clinical data is the aspect of diagnosis requiring the most knowledge and experience. Finally, the remedy picture as found in entries of the Materia Medica is always more comprehensive than the symptomatology that a single individual can ever exhibit. These confounding factors mean that a homeopathic diagnosis remains presumptive until it is verified through testing the effect of the remedy on the patient.