Al-Mustasfa min 'ilm al-usul. (On Legal theory of Muslim Jurisprudence) is Imam Ghazali’s work on the subject of Usul Al Fiqh. It is considered as one of the four great works in the subject. The other three being, 1. The mu`tazalite `Abd al-Jabar (d. 415) al-Qadi's al-`umad; 2. abu al-Husain (d. 473) al-Basri's al-mu`tamad (commentary on al-`umad); 3. al-Imam al-Harmian abu al-Ma`ali (d. 478) Juywani's al-Burhan
Ghazali’s approach to usul al-fiqh, as articulated in this last and greatest work of Law, al-Mustafa, is based on the premise that, in essence, this science is knowledge of how to extract ahkam (rules) from the Shari’ah sources. (As for the science of fiqh, it concerns itself particularly with the Shari’ah rules themselves which have been established in order to qualify the acts of the locus of obligation, man.) Accordingly, Ghazali views it as imperative that any discourse on usul focus on three essential elements: the ahkam; the adilla (sources); and the means by which rules are extracted from these sources, which ultimately includes an examination of the qualifications of the extractor, namely, the mujtahid.