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.
The proving of medicines is a distinctive feature of Homceopathy and a logical necessity for applying the law of cure; for, in order to meet morbid states with drugs corresponding to them, we must know, and therefore ascertain, what morbid states the different medicinal substances produce. It consists in the systematic testing of a drug on the healthy human body, in order to ascertain the changes which it is capable of producing in the functions and organs.
Hahnemann, after viewing the subject in every possible light, and examining every method for ascertaining the action of drugs, came to the conclusion that the only efficient way was "to test the medicines singly and alone on the healthy human body".