the four bases of Islamic Law, Esposito (1988, p. 24) writes of
other sources of law:
While all [fuqaha’]
came to accept the four sources of law, Islamic jurisprudence
recognized other influences, designating them subsidiary
principles of law. Among these were custom [‘urf], public
interest [istislah], and jurist preference or equity [istihsan].
In this way, some remnant of the inductive, human input had
characterized the actual methods of the law schools in their
attempt to realize the Shariah’s primary concern with human
welfare, justice and equity were acknowledged.
In commenting on
supplementary sources of law in Islam, Sardar (1985, pp 114-115)
explains the reason for their importance as follows:
limied parameters, the Shariah is completely open: it can be
developed and shaped according to the needs of society and time
by any number of its other sources: ijma’, qiyas, ijtihad and
istislah. The sources of the Shariah that supplement the Quran
and the Sunnah are problem-solving tools; they provide a
methodology for adjusting to change.