Homoeopathy is derived from the greek word 'Homoios', meaning 'like', as homoeopathy is the medical practice of treating like with like; that is to say, treating an illness with a substance that produces the same symptoms as those displayed by the person who is ill. Choosing a potency: 6c is used when the condition is mainly physical, or when the condition is chronic. 30c is used when the problem is acute or where there is an emotional element.