What is the origin of the jurisprudential maxims: "What is customarily impossible is like what is actually impossible," "It is not denied that rulings change with the changing of times," and "What is tolerated in a subordinate is not tolerated in a principal"?

The first rule states: "Custom is authoritative." Its evidence is the Almighty's saying: (Take what is given freely, enjoin what is right) [Quran 7:199].

The second rule: "Rulings change with the changing of times" applies specifically to rulings based on custom and habit. As for a ruling established by a religious text, it does not change.

The third rule: "What is overlooked in a subordinate is not overlooked in a principal." Its evidence is the permissibility of selling a pregnant she-camel along with its fetus as a subordinate, even though selling the fetus alone is not permissible.