THE LAW OF SIMILARS (similia similibus curentur), or ‘like cures like’ is the central core of homoeopathy. This natural phenomena has been observed since ancient times. The earliest written references that we know of come from Hippocrates, the 'Father of Western medicine'. Others have mentioned their observations of the same phenomena. Until Dr. Samuel Hahnemann (1755-1843), no one had systematically and scientifically experimented with it. He spent 53 years of his life establishing and refining the principles and application of homoeopathic medicine. Dr Samuel Hahnemann, was a German physician, chemist and scholar who in 1790 discovered that doses of a pharmaceutical substance (medicine) that would temporarily cause sick symptoms in a healthy person, cured similar symptoms in a sick person.
The word ‘homoeopathy’ comes from the Greek words 'omoios' (similar) and 'pathos' (suffering/disease).
Conventional medicine works in an 'allopathic' way, i.e. treating with opposites, e.g. diarrhoea is treated with a medicine that would cause constipation in a healthy person. It forces a chemical solution on the body.
Homoeopathy regards sick symptoms as the language of the defence mechanism in it’s own effort to heal the body. Using a small amount of the similar medicine the defence mechanism is stimulated, strengthened and enabled to create order and balance in the sick organism (to cure).  
e.g. 1. Onion juice causes runny nose and weeping eyes = allium cepa (red onion) ameliorates these symptoms in a common cold/allergy.
e.g. 2. Redness and swelling (oedema) with stinging pain as a reaction to an actual bee sting, or other pathology with those characteristic symptoms is cured with a minute amount of apis melifica (bee venom).
e.g. 3. A lot of coffee produces wakefulness, excitability of thoughts and oversensitivity to pain. Coffee in the minute potentised form cures insomnia when the person has excessive mental excitement, especially the joyful kind.
 
For a modern example of the law of similars in a conventional allopathic hospital - read Dr. Nel's mystery.