This document discusses two fiqhi principles:
1) Anything that is a prerequisite for an obligatory act becomes obligatory itself. Examples given include wudu being necessary for prayer, so wudu becomes obligatory.
2) Islamic rulings can change according to changes in time and circumstances. Rulings based on qiyas, public interest, and customary practice may need to change as customs change over time. But rulings proven directly from the Quran and Hadith cannot be changed. Examples of rulings changing with technology and customs are discussed.