The German physician Samuel Hahnemann, born in 1753, made a critical observation in the 1790s when he experimented on himself by taking the drug Cinchona Bark which contains quinine. At the time, Hahnemann was translating a medical text concerning Cinchona from English into German. The English text seemed to contradict his own earlier experience with Cinchona, which he had taken when he was ill with intermittent fever (malaria).
To verify what he was translating, Hahnemann decided to take Cinchona in spite of the fact that he was perfectly healthy. To his interest and surprise, he soon fell ill with symptoms indistinguishable from intermittent fever. He discontinued the medicine and returned to health.
Hahnemann wondered if the reason Cinchona worked against intermittent fever was because it caused very similar symptoms in a healthy person. He gave it to other healthy persons; they, too, fell sick as though with intermittent fever. We can only imagine his excitement at this discovery, one of the most significant in the history of medicine.
Hahnemann continued to experiment (primarily on himself), testing various herbal and mineral substances. Without discriminating between physical, mental, and emotional symptoms, he noted all of the symptoms produced by each medicine in meticulous detail. He found that no two substances produced exactly the same set of symptoms; rather, each caused its own unique pattern of symptoms.
Eventually Hahnemann formulated his Law of Similars, and he began to treat sick people.