Abu Ja‘far, may Allah have mercy upon him, says concern- ing human capacity, that our belief in this (question) is what Imam Musa ibn Ja‘far, peace be upon both of them, said that "A human being has capacity, provided he possesses four characteristics..."
Abu ‘Abdillah adds that what Abu Ja‘far relates from Abu 'l-Hasan Musa, peace be upon him, on al-istita‘ah, is a shadhdh tradition. Al-Istita‘ah, that is, capacity for action, in fact, consists of health and soundness of limb. Thus, every healthy man is a capable agent, and becomes impotent and deprived of capacity only when he is not in possession of health. He would be a capable agent even if he were not in possession of the instrument necessary for the performance of the action.
Then in that case, he is a capable agent impeded from action, and being impeded does not nullify the capacity, but merely the action. In this way a man might be able to marry, though he has not found a woman to marry. Allah, the Exalted, says:
And whoever among you cannot afford to marry free believing women . . . [4:25],
which states that a man is able to marry though he has not yet married, and that he is capable of performing the pilgrimage before doing so, and is capable of going out in the Holy War although he has not done so.
Allah Almighty says:
And they will swear by Allah, if we had been able, we would have gone out with you (Muhammad) [9:42],
which indicates that they were able to go forth, though they refrained from doing so. And He, the Exalted, says:
Pilgrimage to the House is a duty which men owe to Allah, every man who is able to make his way thither [3:97].
Thus, He prescribed pilgrimage for men, which implies that they possessed the capacity for it beforehand. Then how could Abu Ja‘far claim that fornication is dependent on the presence of the woman; whereas we have demonstrated that a man possesses the capacity although he lacks her presence. Hence, if the tradition related by Abu Ja‘far is proved to be sound, then the only meaning of the capacity is to facilitate the action and pave the way for it.
Yet, as we demonstrated, the absence of the means does not involve impotency, and capacity exists in spite of the impediment. Since this subject is a controversial one, and if we deal with it at length it will be long drawn out indeed, then what we have established about it is sufficient to the intelligent.