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Istislah (Arabic استصلاح "to deem proper") is a method employed by Muslim jurists to solve problems that find no clear answer in sacred religious texts. It is related to the term مصلحة Maslaha, or "public interest" (both words being derived from the same triconsonantal root, "ṣ-l-ḥ"). Extra-textual pragmatic considerations are commonly accepted in Islamic jurisprudence concerning areas where the Qur'an and the practices of the earliest Muslim generations provide no specific guidance. However, appeals to Istislah or Maslaha are controversial when the goal is reforming what has been considered to be divinely-revealed law.
Istislah bears some similarities to the natural law tradition in the West, as exemplified by Thomas Aquinas. However, whereas natural law deems good that which is known self-evidently to be good, according as it tends towards the fulfilment of the person, istislah calls good whatever is connected to one of five "basic goods". Al-Ghazali abstracted these "basic goods" from the legal precepts in the Qu'ran and Sunnah: they are religion, life, reason, lineage and property. Some add also "honour".
Istislah, in this classical formulation, is not mere utilitarianism, which calls good whatever brings about "the greatest happiness for the greatest number." That is so because a measure bringing about the "greatest happiness" might infringe any one of the five basic values.
A more "liberal" strain of istislah has been important in the twentieth century and centres on the work of Rashid Rida. Rida considered that the "no harm no retribution" hadith is a supreme principle of legal liberalism, before which all other principles of the Shari'ah must give way. By this method, legislation promoting negative freedoms and human rights is to be considered "Islamic". In Egypt this approach has been upheld by the Supreme Constitutional Court, which has ratified equitable measures benefiting women even where these seemingly conflict with principles of classical Shari'ah.
- Mawil Izzi Dien. Islamic Law: From Historical Foundations to Contemporary Practice. p. 69.