What is the difference between Uṣūl al-Fiqh (Principles of Islamic Jurisprudence) and Qawāʿid al-Fiqh (Maxims of Islamic Jurisprudence)? Is it permissible for a student of the Hanbali school to study Uṣūl al-Fiqh written by a Shafi‘i or Maliki author, or Qawāʿid al-Fiqh from another madhhab?

Studying a specific school of thought (madhhab) from its own books is the most suitable approach for understanding and mastery. However, it is not a religious obligation such that deviating from it would be impermissible. Therefore, it is incorrect to say that it is impermissible for a Shafi'i student to study the rules of Hanbali jurisprudence, as no scholar holds this view. Rather, studying any school of thought requires knowledge of its rules and principles, and for that reason, the student should turn to the books of the school's principles (usul) and rules (qawa'id). The principles of jurisprudence (usul al-fiqh) among the three schools: Maliki, Shafi'i, and Hanbali, are similar, which explains why some of their scholars pay attention to books on principles from other schools.