Similia Similibus Curanter
This is the law of similars.
It states that 'that which can cause can cure'. The onion, which produces tears in the eye and irritation (similar to a cold), can be used as a homeopathic medicine to cure colds which have irritating tears. The early Indians recognised this principle and states that Vishasya Vishamevam Aushadam and Samaha Samena Shantihi, but it was Dr.Samuel Hahnemann, who through his studies and experiments on the various medicines available in nature, practically proved the law.