For those familiar with the science of usul al-fiqh there is a well known statement by imam Malik bin Anas – may Allah have mercy on him – on istihsan related by Ibn al-Qasim that he said that istihsan (discretion) was nine-tenths of knowledge. Judgements based on istihsan or which make it the deciding factor when weighing up different proofs are numerous in the Maliki madhhab according to ash-Shatibi rahimahullah. One example of this is loans. Often it is heard loans are haram just as riba is. A loan may indeed be considered as riba because one dirham, for example, or one euro is exchanged for one euro for a period of time but it is permissible under the principle of istihsan because of the way people are helped by it. If loans had remained forbidden people might have suffered great hardship and our din after all is yusr: ease, as the famous hadith goes.
Also ash-Shatibi said that istihsan is the foundation of knowledge.
(Source: M. Abu Zahra, The Four Imams, pp. 103, 104)