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.
This is the only scientific and rational method of ascertaining the action of medicines. All other methods have proved useless and misleading. The credit of first teaching the necessity of proving drugs upon the healthy belongs to Albrecht von Haller, a Swiss physician. As early as 1755, in his Swiss pharmacopoeia, he distinctly taught this, but nothing came of it. It was not until Samuel Hahnemann, in 1796, practically went to work and actually experimented with drugs on himself and others that the first pure effects of drugs became known and could be rationally employed in practice. Hahnemann was thus the founder of the science of drug pathogenesy, for it is a fact that up to his tinterio one had made any physiological experiment with any drug; it is a fact that his experiments with Peruvian Bark were the first ever made in the domain of pharmacology, and are a model to this day. The science of drug proving dates therefore from 1796, and is the beginning of a rational therapeutics. With the adoption of this principle, we have a key to unlock the sphere of action of every substance in nature, so far as it can bear any relation to man's constitution in health and disease. It is a creative principle, universal in its application, true for all time. The introduction of it into medicine marks an epoch in the development of the healing art, before which time a science of therapeutics was impossible.
In the Organon, §§ 108-9, Hahnemann says: "There is, therefore, no other possible way in which the peculiar effects of medicine on the health of individuals can be accurately ascertained; there is no sure, no more natural way of accomplishing this object than to administer the several medicines experimentally, in moderate doses, to healthy persons, in order to ascertain what changes, symptoms and signs of their influence each individual produces on the health of the body and of the mind; that is to say, what disease elements they are able and tend to produce, since, as has been demonstrated, all the curative power of medicines lies in the power they possess of changing the state of man's health, and is revealed by observation of the latter".