Sacred laws change according to the ages. Indeed, in one age different prophets may come, and they have come. Since subsequent to the Seal of the Prophets, his Greater Shari'a is sufficient for all peoples in every age, no need has remained for different laws. However, in secondary matters, the need for different schools has persisted to a degree. Just as clothes change with the change of the seasons and medicines change according to dispositions, so sacred laws change according to the ages, and their ordinances change according to the capacities of peoples. Because the secondary matters of the ordinances of the Shari'a look to human circumstances; they come according to them, and are like medicine.
At the time of the early prophets, since social classes were far apart and men's characters were both somewhat coarse and violent, and their minds, primitive and close to nomadism, the laws at that time came all in different forms, appropriate to their conditions. There were even different prophets and laws in the same continent in the same century. Then, since with the coming of the Prophet of the end of time, man as though advanced from the primary to the secondary stage, and through numerous revolutions and upheavals reached a position at which all the human peoples could receive a single lesson and listen to a single teacher and act in accordance with a single law, no need remained for different laws, neither was there necessity for different teachers. But because they were not all at completely the same level and did not proceed in the same sort of social life, the schools of law became numerous. If, like students of a school of higher education, the vast majority of mankind were clothed in the same sort of social life and attained the same level, then all the schools could be united. But just as the state of the world does not permit that, so the schools of law cannot be the same.