It was therefore natural for the messengers of truth to convey their message in the language of the land wherein they were born, and in the style suited to the life of their period. For each one was needed in his place and adapted to his era, and the difference between them existed only in those principles and rituals which were given to the people of that time and harmonized with their standard of intelligence and evolution: even as a physician has to change his prescriptions according to the patient's state of improvement before he can bring about the cure; or as in school, at each term and in every year, a new course of study is taught through different grades.