Homeopathy works on the principle of the "Law of Similars". This principle has been part of medical practice since the time of the Greeks. Although Homeopathy has been developed and refined over 200 years, but it was first formulated by Samuel Hahneman, a German physician in the eighteenth century.
Hahneman postulated the law which states "that, which makes sick, shall heal". In other words, a substance, which causes particular symptoms in a healthy person, will cure those same symptoms in an ill person.
An example of this can be illustrated by the physical symptoms caused by peeling an onion. The symptoms include an acrid runny nose, stinging streaming eyes and a particular soreness in the throat.
"Allium cepa", the name of the homeopathic remedy that is made from onion, is one of the remedies used for the treatment of hay fever when these are the presenting symptoms.