Ilm al Khilaf a science which deals with the Islamically sound arguments used by Muslim jurists (fuqaha) to reach their various legal opinions. As such, it can also be known as comparative Islamic law. Historically, the various madhahib of Islam shared and benefitted from this science, and there is no reason why we also cannot benefit from it.
We know that there were differences of opinion and practice even during the time of the Prophet Muhammad, for his Companions did not view everything the same way. This state of affairs naturally continued after the Prophet’s death. The suhuf ascribed to some of the Companions and the information given in the relevant biographical literature were studied for details. The fiqh-related issues debated by the Companions increased in number and complexity during the time of the Successors (Tabi’un),w hen the development of Islamic jurisprudence was just getting started.
The judicial and juristic opinions expressed in Majmu al Fiqh by Zayd ibn ‘Ali (d. l22/740), in al Muwata’by Imam Malik (d. 179/795), by Muhammad ibn al Hasan al Shaybani (d. 189/805) which he ascribed.to his master Abu Hanifah, and by Abu Yusuf Ya‘qub (d. 182/769), especially in his Kitab al Kharaj, al Radd ala Siyar al Awzi, and Iktilaf Abi Hanifah waIbn Abi Layla may be cited here as the first scholarly sources of ‘ilm al khilaf. The related literature on differences between the fuqaha’ is full of points which may be studied for further information.
With the spread of Islam and the incorporation of many non-Muslim subjects into its domains, new legal questions were raised. The resulting decisions led to the profound and scholarly development of ilm al khilaf and the establishment of the methodological principles of Islamic law in several branches. However, it was not until the beginning of the fifth hijri century that comparative Islamic law was scientifically established as an independent branch of Islamic law by Abu Zayd ‘Abd Allah ibn ‘Umar ibn ‘Isa.