A thing required to fulfill a wājib can be of two types: i) it can be such that the obligation is conditional upon it, or ii) it can such that the obligation is not conditional upon it. If the obligation is conditional upon it than there is no difference in that fulfilling that condition is not a wājib. Rather the wājib is that whose obligation is established by the evidence. An example of this is the obligation of a specific salāt, which is conditional upon the attainment of purity. Yet the purity is not an obligation from the perspective of the address which establishes the obligation of the salāt. Rather it is a condition for the performance of that obligation, and that obligation in the address of salāt is but that salāt if the condition exists. As for the thing required to fulfill a wājib but not as a (legal) condition, rather as a condition of reality, then this is of two types: i) that which is within the ability of the mukallaf and ii) that which is not within the ability of the mukallaf. As for the former, than it is wājib by the same address which establishes the original required wājib, and its obligation is entirely like the obligation of the matter brought by the address of the Legislator. An example of this is washing the elbows [mirfaqayn], without which the wājib of washing the arms up to the elbows cannot be accomplished. Accomplishing it depends on washing at least a part of the elbows; hence washing a part of the elbows is wājib, even though the address did not expressly state this. This is because the address came with that whose accomplishment depends on this, hence that address includes the original wājib as well as that which its performance depends upon. The indication of the address upon this later obligation is an implicative indication [dalālat al-iltizām]. Another example is the establishing of a political structure in order to establish the Khalīfah when one does not exist, or to account the rulers. Establishing the Khalīfah is an obligation, as is accounting the rulers. Fulfilling these obligations properly is not possible by individuals alone, and hence requires the structuring of a group of the Muslims who as a collective are able to fulfill them. Thus it becomes wājib upon the Muslims to establish a group able to establish the Khalīfah or account the rulers. If they do not do this they are all sinful, because they did not fulfill that which was required for the fulfillment of a wājib. And if they establish a group but one unable to fulfill the required tasks they are still sinful and have not fulfilled the obligation. This is because the obligation is not to merely establish a group, but to establish a group able to fulfill the required obligation.

As for that which is not within the ability of the mukallaf, then it is not wājib, because of the saying of Allah,

“Allah does not burden a soul with more than it can bear…” (al-Baqarah: 286); and his (saw) saying, “…and if I command you to do a thing than do of it as much as you are able” (Bukhari, Muslim), and because it is not permissible to make one responsible for that which is beyond their ability, as this necessitates the attribution of oppression to Allah, and this is not permissible.

In sum, the thing required to fulfill a wājib is itself wājib, either by the same address which establishes the original wājib or by another address. This is irrespective of whether this thing is a sabab [legal cause] – that whose presence necessitates the presence (of the hukm) and whose absence necessitates its absence – or a shart [legal condition] – that whose absence necessitates the absence (of the hukm) but whose presence necessitates neither its presence nor absence. It is also irrespective of whether the sabab is legal [shar’i] like the required tense in relation to the obligatory emancipation (of a slave), rational, like the (intellectual) assessment which leads to obligatory knowledge, or normal [‘ādī] like the incision of the neck in the obligatory slaughtering. Similarly it is irrespective of where the shart is legal, like the wudū, rational - that which is a rational necessity for what is commanded - like leaving the opposite of what is commanded, or normal – that which is required as a matter of usual occurrence – like washing a part of the head in wudū. Hence the obligation of a thing obligates that which it cannot be fulfilled without, that is, legal responsibility to fulfill a thing necessitates the legal responsibility of that which it cannot be fulfilled without. And thus we arrive at the principle, “that which is required to fulfill a wājib is itself wājib.”