In homeopathy, what is the law of similars?

The law of similars is fundamental to the practice of homeopathy. This tenet states that a substance that causes a certain set of symptoms in a healthy person will have a curative effect when given to a sick person who is displaying similar symptoms. "Let likes be cured with likes," is the phrase termed by Samuel Hahnemann, the scientist who founded homeopathy over 200 years ago. In conventional medicine, this same concept is used in the development of vaccinations. Small doses of the agents that cause the disease are used to immunize the patient against the disease. Vaccines are sometimes compared to homeopathic nosodes, but while the theory behind both is similar the method of development and administration are very different.