Al Mustasfa Min ilm Al Usul : المستصفى من علم الأصول

Al-Mustasfa min ilm al-usul. (On Legal theory of Muslim Jurisprudence) is Imam Ghazalis 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-Qadis al- umad;

2. Abu al-Husain (d. 473) al-Basris al-mu tamad(commentary on al- umad);

3. Al-Imam al-Haramain abu al-Ma ali (d. 478) Juywanis al-Burhan

Ghazalis 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 Shariah sources. (As for the science of fiqh, it concerns itself particularly with the Shariah 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 examination of the qualifications of the extractor, namely, the mujtahid.