The principle of 'like can cure like' ( an illness should be treated by a substance capable of producing similar symptoms to those being suffered by the patient) dates back to the Greek physician Hippocrates (460-377 B.C) in the 5th century B.C. He was the first person to think that  disease was the result of natural forces, not divine influences. He believed that careful observation of the symptoms specific to an individual and the persons reactions and his own powers of healing should be taken into account before reaching a diagnosis and choosing a cure. Hippocrates known as the 'father of Medicine' had a collection of several hundred remedies. One of the best examples he provided of the principle of 'like curing like' was using the root of Veratrum album (white hellebore) in the treatment of cholera. In large doses this highly poisonous root causes violent purging that leads to severe dehydration, causing the same symptoms of cholera.

In the early 16th century, Swiss doctor Paracelus (1493-1541) found out that the causes of diseases were linked to external forces such as contaminated  food and drink. He also believed that a poisonous substance that causes disease could also cure the disease' if given in very small doses and that physicians should take into account the body's own natural ability to heal itself. Here again the principle of 'Homeopathy' was advocated. But it did not gain popularity for another 300 years, until Homeopathy came into being. 

In 1790, while translating  'A treatise on Materia Medica' by Dr. William Cullen, Hahnemann came across a passage about Peruvian Bark or Cinchona. It stated that quinine which is a substance purified from the bark of the cinchona tree, was a good treatment for malaria because of its astringent qualities. This made no sense to Hahnemann who, as a chemist was aware that there were other much more powerful astringents that had no effect on malaria. Deciding to investigate further, he dosed himself with quinine and recorded his reactions in great detail. He begun to develop the symptoms of malaria one after another, despite the fact that he actually did not have the disease. The symptoms recurred every time he took a dose of quinine and lasted for several hours. If he did not take any quinine, he had no symptoms. He repeated the doses of quinine, which he called 'provings' on people he knew well, noting the reactions in great detail. He then repeated the process using other substances such as arsenic and belladonna under strict conditions. The 'provers' were not allowed to eat or drink anything that might confuse the results such as alcohol, tea, coffee and salty or spicy foods. The provers  response varied, some showed a few mild symptoms to a particular substance, while others experienced vigorous reactions with a variety of symptoms.

  The symptoms that were most commonly found for each substance he called first line or keynote symptoms. Second line symptoms were less common and third line symptoms were rare. The combination of symptoms made up a 'drug picture' for each substance. He continued to conduct experiments for 6 years, testing a wide range of substances. He compiled the 'drug pictures' he had collected from his careful research, and started to test each substance on the sick to see whether they benefited from it. The patients were physically examined and thoroughly questioned about their symptoms, i.e., their general health, the way they lived and their outlook on life and what factors made them better or worse. Thus he build up a symptoms picture of each patient. Then he matched the individuals symptoms picture to the 'drug picture' of various substance. When he established the closest match, he would prescribe a remedy. He found that the closer the match, the more successful the treatment. Thus a new system of medicine 'Homeopathy' was discovered.

  In 1776, Hanhemann published his book 'A new principle for ascertaining the curative powers of drugs and some examination of previous Principles', his first work on Homeopathy. In this book, he explained the key principle that, a drug taken in small amounts will cure the same symptoms it causes in large amounts.