Hanifa, of the Sunni Hanafi school of fiqh believed that the six commodities shared the common feature (`illah) of being able to be weighed or measured, so that other commodities sold by weighing or measuring were subject to the same rule. Imam Al-Shafi‘i, of the Shafi'i school of fiqh, was of the opinion that their common feature (`llah) was that they were either eatables or were used as a universal legal tender. Thus, to him, all eatables and universal legal tenders were subject to riba al-fadl. For Imam Malik ibn Anas of the Maliki school the common feature of the six was that they were either food items or could be stored (i.e. were non-perishable), so in this school only food items or storable items are included in this category. This disagreement (according to Taqi Usmani) is the part of the lament of Rashidun Caliph Umar that Muhammad did not explain the prohibition more clearly. Criticism Critics of this interpretation include activist Khalid Zaheer and economists M.A. Khan and Mohammad Omar Farooq. Zaheer believes that "the literature on Islamic Finance and Economics is presenting very strange applications of the concept of riba al-fadl, which are ... being applied in areas of business and finance where their application was never intended." He notes that some scholars "openly" admit they do not understand the logic of the ban on Riba al-Fadl. Prohibition of riba al-fadl (specifically in barter of six specified commodities) is mentioned only in hadith. M.A. Khan and Farooq find the reference to riba al-fadl questionable as it makes no sense. Khan asks why anyone would ever trade equal quantities of the same kind of commodity ("like for like") — for example 100 kilograms of wheat for 100 kg of wheat — in a riba-free transaction called for by quoted ahadith. Or how "divine law" could prescribe that a jeweler — "who has spent his time and effort to convert gold into jewelry" and is taking gold as payment — not be compensated? M.A. Khan also notes that the authors of the IIIE blueprint have no objection to traders selling higher purity/quality commodity for cash and using the proceeds to buying more less purity/quality commodity, and wonders what would be accomplished by such "an ineffective and roundabout method of handling a simple exchange transaction". Abdullah Saeed complains that the legal cause or feature (`illa) used by the schools of Islamic