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.
Drug Pathogenesy is the record of testing drugs on the human body in varying doses, and on different individuals of both sexes, and observing all the symptoms, subjective and objective, from the minutest disturbed function and mental state to the grossest organic lesion. "Simple drugs," says Hahnemann, "produce in the healthy body symptoms peculiar to themselves, but not all at once, nor in one and the same series, nor all in each experimenter". Such a method of arriving at a knowledge of drugs is universal in its application; it includes all that can be learned from toxicology also.
The value of toxicology as illustrating drug action is far inferior, however, to that of testing them in health by means of small doses. Still it gives the ultimate lesions and organic changes, and in this way-interprets many symptoms of the provings; but whenever the organism is violently invaded by a foreign destructive agent, no matter what the poison is, there is usually much similarity of action, resulting from nature's efforts to throw it off by all possible routes outward from the body's distinctive vital centers, hence the inevitable nausea, vomiting, diarrhoea, nerve disturbances, convulsions, paralysis, etc., of most poisonings. The provings with small doses avoid these crude, extreme effects, and, instead of producing them, rather indicate them by the milder disturbances produced.