The point here is to distinguish between that without which an obligation does not arise and that without which an obligatory act cannot be fulfilled. Our discussion concerns the latter. Whatever is indispensable to fulfilling an obligation—such as travelling the necessary distance to attend the Friday prayer or perform Hajj, and the like—must be done by the legally accountable person, by agreement of all Muslims.
However, someone who neglects Hajj while living far from Mecca, or neglects the Friday prayer while living far from the congregational mosque, has omitted more than someone who lives nearby. Nevertheless, it is not said that his punishment is greater than that of someone living nearby. An obligatory act is one whose omission is grounds for blame and punishment. If something that must be done as a subsidiary requirement were itself intended as an obligation, omitting it would incur greater blame and punishment. Thus, someone from India or al-Andalus who neglected Hajj would be punished more severely than someone from Mecca or Tā’if who neglected it; and someone from the farthest part of the city who neglected the Friday prayer would be punished more severely than someone living beside the congregational mosque who neglected it.
Since it is known that someone living farther away receives a greater reward, yet, if he neglects the obligation, his punishment is no greater than that of someone living nearby, the following difficulty arose: is this prerequisite obligatory or not?
The precise answer is that it is obligatory by logical necessity, not because the command-giver intends to require it. Rather, in commanding an act, he may not intend to require its necessary prerequisites, even though he knows that they must be present. If he is someone liable to heedlessness, those prerequisites may not even cross his mind.
Whoever understands this will find al-Kaʿbī’s difficulty resolved: does Islamic law contain anything permissible or not? Al-Kaʿbī claimed that nothing in Islamic law is permissible, because there is no permissible act that a person performs