The principle of "like cures like" can be found in the writings of the "father of medicine," Hippocrates, which date from the 5th century s.c., and has been echoed through the ages in folk cures, such as rubbing frostbite with snow.   A German doctor, Samuel Hahnemann, rediscovered this principle in the late 18th century. Rejecting leeches, violent purges, and other severe medical practices of the day, he developed homeopathy, taking the name from the Greek homoios (same) and pathos (suffering). His starting point was the contemporary use of quinine to treat malaria, allegedly due to its bitter qualities. Using himself as a guinea pig, he took regular doses of quinine and developed malaria-like symptoms. He concluded that it was actually quinine's ability to cause a malaria-like reaction that made it effective against the disease. He then persuaded healthy people to test, or "prove," other substances, such as arsenic and belladonna, and began to use these "remedies" to treat illnesses whose symptoms resembled the effects the substances had produced.  

Current scientific understanding cannot explain homeopathy. While accepting that minute quantities of a substance can affect physiological processes, many doctors are skeptical of the efficacy of a substance that is so diluted that barely a molecule remains in the remedy. A 12c potency, for example, is said to be equivalent to a pinch of salt in the Atlantic Ocean.   The popular but controversial theory of "water memory," which needs further research, contends that even though molecules have been diluted away, they leave electromagnetic "footprints" in the solution, to which the body responds.