According to Professor Ahmed Ibrahim, as he stated in his book علم أصول الفقه, Preference, الاستحسان, is defined as one of two things,
- An analogy whose contingency (العلة) is not clear in preference to another analogy whose contingency is clear, or
- An analogy whose contingency leads to the opposite of the preferred conclusion.
Such preference is accepted in the Hanafi school of thought as a legitimate deduction method. Ash-Shaafii strongly disagrees and has famously said, "من استحسن فقد شرع" (Whoever prefers has legislated).
As examples of preference, Professor Ibrahim mentions that a bequest is considered invalid by analogy, because it's gifting money that is no longer the property of the gifter. However, preference is used here because authentic text allowed it.
I respectfully disagree on both counts. The analogy is flawed because the testator was alive when he made the bequest and therefore the bequest was from his property. Furthermore, it is not a preference to yield to authentic text; preference is to yield to an unjustified analogy.
Professor Ibrahim comments on Ash-Shaafii's opinion of preference that there is good preference and there is bad preference. But the question is: what are the guidelines that distinguish the two? By definition preference is based on invalid analogy, so how can anyone judge its applicability to the issue at hand? And who can determine those guidelines, and what happens when two scholars disagree on those guidelines? This leads to a mess and confusion.
I agree with Imaam Ash-Shaafii on rejecting preference as a legitimate deduction method.