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.
Drugs are substances taken from all the kingdoms of nature that are used as medicines. They usually produce deleterious effects upon the body when given in sufficiently large doses in health, and they modify some part, or the whole, of a morbid state when given to the sick. This property is discovered either empirically by observation of cases of poisoning, or by systematic experiments on human beings in health.
Medicinal force is a distinct property of drugs, by which they modify vital activity, not by their chemical, physical or mechanical properties, but by their specific dynamic property, peculiar, distinctive and different in every drug. And they can be truly curative only by reason of their modifying properties of the vital processes. Each medicinal substance, be it plant, mineral or animal product, has stored within its material particles, and embodies, therefore, its own particular medicinal force, which can be brought into activity by breaking up the outward particles. The character of this specific force, or the drug's medicinal properties, can only be discovered by the vital test, made by experimenting with different doses on healthy human organisms, and to some extent on animals. The latter merely to see the ultimate lesions and organic changes drugs are able to produce.
At one time, and especially during the middle ages, before the introduction of modern, scientific methods, the properties of drugs were based upon the Doctrine Of Signatures.
External characteristics of a substance served to indicate possible therapeutic effects. Fancied or real resemblances between some part of a plant and some particular organ or fluid of the body pointed to therapeutic relationship. Thus, according to this doctrine, Digitalis must be of use in blood diseases, because its flowers are adorned with blood colored dots; Euphrasia was famous as a remedy for the eyes, because it had a black spot in its coralla, which looked like a pupil. *
The lungs of a fox must be specific against asthma, because this animal has a very vigorous respiration.
Hypericum having red juice ought therefore to be of use in haemorrhages.
Euphorbia, having a milky juice, must be good for increasing the flow of milk.
Sticta, having some likeness to the lungs, was called pulmonarius and esteemed as a remedy for pulmonary complaints.
Singularly enough, in isolated instances at least, such relationship actually does exist, as has been verified by subsequent clinical application, and it is possible that an intuitively gifted race may see a relationship actually existing between outward forms and structures and inner uses. But for purposes of modern scientific therapeutics it is valueless. *
Other methods of determining the medicinal virtues of drugs were by the sense of taste and of smell. Drugs with a bitter taste were held to possess tonic and stomachic virtues, hence the "Bitters" of the ordinary pharmacopoeia. Gentiana, for instance, is such a drug, and unquestionably does exert a tonic influence upon the stomach. But this virtue is probably not because it is bitter, but because it has a distinctive medicinal force wholly independent of its taste. Certain aromatic drugs were deemed to possess anti-spasmodic and stimulant properties, etc.