and the Prophet’s mention of these particular things was merely by way of example, since they were what was available at the time. This is like the charity of fiṭr, concerning which he said: “A sāʿ (a standard measure) of wheat and a sāʿ of barley” (1), even though there are other things as well. But what is the basis of the ruling—that is, what is the precise effective cause by which other items may be included alongside these six categories? This too is a matter of dispute.

Some scholars said that the effective cause is measurement by volume and by weight, because these things are either measured by volume or by weight: volume applies to the four categories, and weight to gold and silver. This is the well-known position in the school of Imam Ahmad. Accordingly, we say that ribā applies to everything measured by volume, by analogy with the four categories, and to everything measured by weight, by analogy with the other two categories, gold and silver. Ribā does not apply to anything that is neither measured by volume nor by weight, and it is not conditional that the item be edible; rather, it applies even if it is not eaten. Thus, ishnān (a non-edible alkaline plant) is measured by volume and is not eaten, yet ribā applies to it because it is measured by volume.

If one orange is exchanged for two oranges, this is permissible, since oranges are neither measured by volume nor by weight, but are counted items. The proof is the hadith: “Gold for gold, silver for silver, wheat for wheat.” Let us consider whether the evidence corresponds to what is being inferred, or whether what is being inferred is broader. It is known that if what is being inferred is broader, then the argument is invalid, because a more specific proof excludes everything other than what it specifically addresses. But if the proof is broader and we use it to establish a narrower ruling, this is permissible, because the narrower case is one of the instances included in the general wording. This is a principle of legal argumentation: whenever the proof is more specific, it is not valid to use it to establish a broader ruling; conversely, the reverse is valid.