Homeopathy was founded in the late 18th century when Samuel Hahnemann, a German physician, translated a thesis on the curative properties of Peruvian bark in the treatment of malaria. His curiosity was aroused by the theory that the bitterness of the bark was the medicinal agent. Hahnemann decided to test this claim by taking  doses of Peruvian bark over a period of days to see what happened. After repeated doses, he produced the symptoms of intermittent fever normally associated with malaria. When he stopped taking the bark the symptoms disappeared.

Hahnemann realised this effect could be the reason Peruvian bark cured the fever of malaria: because it could cause similar symptoms. Hippocrates, two thousand years before Hahnemann, knew about this principle, saying ‘by the administration of the similar medicine shall the patient be cured’.

Hahnemann experimented with this idea with several well-known toxic agents such as arsenic, mercury, antimony and copper. Hahnemann conducted trials with these drugs, in increasingly diluted forms, to discover the symptoms they caused in healthy humans. He discovered how to potentiate, or potentise, extremely dilute solutions. He discovered that the medicinal power to heal was not diminished by dilution if the total symptom picture of the sick person was very similar in nature to that of the medicinal agent. These experiments led Hahnemann to the Principle of Similars.

See How Homeopathy Works  for further information.

Also see Principle of Similars