Refutation may be directed against a definition, a proof, a universal proposition, or a legal cause. With proofs and universal propositions, however, converse entailment is not required unless an independent consideration is involved—for example, when two concomitant matters are employed and one is cited as evidence for the other; in such a case the proof must work both ways. A definition, by contrast, must always exhibit converse entailment.

When the legal cause (ʿillah) is complete, forward entailment is obligatory; if it is incomplete, the ruling may fail to follow because a condition is lacking or an impediment is present. This is called “qualification” (takhṣīṣ) and “refutation” (naqḍ). The occurrence of the ruling in the absence of its cause is termed “lack of converse” and “lack of effect.” The ruling linked to that cause does not necessarily disappear merely because the cause is absent, nor must its counterpart disappear when another cause is operative. Mere failure of converse entailment, therefore, does not invalidate a cause unless the ruling is found without the cause and without any other cause taking its place—something that is, in reality, simply another instance of the same species. In that situation the cause is ineffectual and therefore null. Thus lack of effect invalidates a cause, whereas lack of converse does not. When the cause is absent the ruling dependent upon it ceases, and when the ruling arises from another cause, then if that cause is identical to the first it is in fact the same cause, but if it is different the ruling must likewise differ.

“Permissibility of bloodshed” is therefore a generic label: the permissibility that follows apostasy is one species, that which follows adultery another, and that which follows homicide yet another. The permissibility that results from homicide allows for ransom and pardon; the permissibility that results from apostasy allows for forgiveness through repentance; whereas the permissibility that results from adultery entails the prescribed penalty of stoning to death in the presence of witnesses—and similar examples are numerous. Hence, when two instances of permissibility are identical, the two causes are one; but when the rulings differ, the causes must also differ. The nullification of ablution by urine and excrement constitutes a single kind and is not an example of divergent rulings produced by two causes, as is the case with bloodshed under the various legal penalties.

The objector says:

“These attributes have no effect on the permissibility of bloodshed, for apostasy, the adultery of a muḥṣan, and the armed hostility of an original unbeliever all render bloodshed lawful even when these attributes are absent.”

We reply: The permissibility in these cases arises from a different cause, not because these attributes are ineffectual—just as a son acquires his entitlement for a separate reason.