Like Cures Like
The word homeopathy is coined from the Greek words 'homoios' meaning like or similar and 'pathos' meaning suffering (similar suffering). It works on the principle of 'similia similibus curentur', or let 'likes be cured by likes' which means, that which will cause will cure.
According to the Law of Similars, a substance that when taken in crude form causes a set of symptoms or disease in a healthy person can cure similar symptoms occurring during an illness when treated with small, often infinitesimal, potentised doses of the same substance. This substance will help to stimulate the body's own healing power to effect a deep and long-lasting cure. The concept has been around since Hippocrates in 450 BC. Dr. Samuel Hahnemann in the 18th century developed homeopathy further, through provings and scientific examination.
Based upon the homeopathic principle, suppose a person has hay fever, with watery eyes and a burning nasal discharge. Instead of giving an antihistamine to dry up the discharge, a homeopath might prescribe Allium cepa - a tiny dose of onion specially prepared by a homeopathic pharmacy - because onion is known to cause watery eyes and a burning nasal discharge. Similarly, suppose a person can't sleep because of agitation or an overactive mind, instead of giving a conventional "sleeping pill," a homeopath might give Coffea - a homeopathic preparation of coffee. Along the same lines, someone experiencing nausea and vomiting would receive a homeopathic medicine made from a substance that would normally cause nausea and vomiting in a healthy person when taken in a crude form.