This section is from the book "A Compend Of The Principles Of Homoeopathy", by William Boericke. Also available from Amazon: A Compend of the Principles of Homoeopathy as Taught by Hahnemann.
Every drug affects some organ or system of organs or tissue or region more decidedly than others, and there especially or primarily expends its power. This is not a local action merely, but a localization of the drug's specific nature. It appears, no matter by what avenue the drug is introduced into the body. A drug may come into direct contact with the blood, and thereby with every part of the organism, and yet only certain tissues or organs will be affected by it - that is, only these tissues or organs will react against the foreign element. This specific localization, or specificity of seat of a drug, is known as its elective affinity, by which it preferably chooses certain cells, tissues or organs, to manifest its action. In a general way, we see that Belladonna affects principally the brain as its arena for action, and this organ, therefore, has a preferred relationship to Belladonna. So, in the same way, Aconite affects the heart, Ergot the uterus, Bryonia the serous membranes, Podophyllum the duodenum, Rhus the skin; Tellurium, the tympanum; Glonoin the vaso-motor centre in the brain; Phosphor, the periosteum.
This elective affinity cannot be explained, but it exists. It was recognized even before Hahnemann and homoeopathic provings, and has been made the foundation of a system of practice by Rademacher, a German physician and contemporary of Hahnemann, who himself traces the thought to Paracelsus.
While each drug has a preferred locality, based on its elective affinity, still it must not be forgotten that the whole organism - the whole man, mentally and physically, - is affected. This is so, because the various functions and organs are not independent instruments, but wonderfully bound together by nerves and blood vessels, and parts most remote are in direct nerve communication with each other. Diseases are produced and continued along these lines of network, when once they have found a foothold, and drugs act in a similar manner along these tracks. We ought to get a mental picture of a drug as a whole - the drug personified, and thus the typical patient corresponding to the drug. Such study gives a reality and practical utility to the Materia Medica.