In the Name of Allah, the Beneficent, the Merciful
The subject under discussion is the knowledge of the Principles of Jurisprudence (Usul’ul Fiqh). The study of Principles of Jurisprudence is tantamount to a preparation to the study of Jurisprudence.
The knowledge about the Principles of Jurisprudence is more profound than the knowledge of Jurisprudence itself. The relationship between the study of Jurisprudence and its Principles is the same as it is between the study of Logic and Philosophy.
For example, everybody knows that the price of a certain commodity shows an upward trend while that of another remains static. This knowledge is superficial but the knowledge as to why the prices show an upward trend is a deep-rooted knowledge. The Holy Qur'an and the Sunnah of the Holy Prophet gives us precise commandments and edicts to adhere to the teachings of Islam in every walk of life; but all of these commandments have not been explained in detail.
It has been so because events and situations pertaining to relevant human activity and behavior vary; but to arrive at conclusions regarding various general rules and regulations, a guideline in the form of principles has no doubt been laid down.
Hence, the study of the Principles of Jurisprudence viz. the principles of deducing laws has become very important as well as a fascinating subject. The work on this subject started from the second century of the Hegira with a view to making correct deductions from Islamic injunctions for practical purposes.
In short, the Principles of Jurisprudence is the study of those rules that are used in deducing Islamic laws from the Book of Allah (Qur'an) and the Traditions of the Holy Prophet (AHadith).