Why are there more than one school of law?

Laws change according to ages. Prior to the Seal of the Prophets, at times it happened that in one age different Prophets came to different peoples with different laws. After the Seal of the Prophets, upon him be peace and blessings, however, since his most comprehensive Shari'ah is sufficient for all the peoples in every age, no need has remained for other laws. Nevertheless, in secondary matters of the Shari'a, the need for different schools has remained to a degree. Just as clothes change according to seasons and cures may differ according to temperaments, so too with the change of ages and according to the characters and capacities of peoples, the rules for the secondary matters of the Shari'a may differ. Because those secondary matters are concerned with human temperaments and the prevailing conditions of the time, they come according to them and become like cures.

At the time of the early Prophets, peoples were distant from one another both physically and with respect to the level of education, and their characters were somewhat coarse and violent, and their minds, primitive. For this reason, the Laws which came at that time were all different and appropriate to their conditions. There were even different Prophets and Laws in the same region in the same age.

Then, since the Last Prophet came with an all-comprehensive religion to lead mankind to pass to the secondary and further stages in science, education and civilization, and to enable them, through numerous revolutions and upheavals, to reach a position at which all 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 any need for different teachers. But because they have never been all at completely the same level and led the same sort of social life, there have been different schools of conduct. If, like the students of a school of higher education at al-most the same level with one another, the absolute majority of mankind were to lead the same sort of social life and attain the same level, then all the schools could be united. But since the conditions of the world do not allow that, the schools of law cannot be the same.