The shaykh Abu Ja'far, may Allah have mercy on him, said: Our belief regarding this (question) is what Imam Musa bin Ja'far al-Kazim, on both of whom be peace, said, when he was asked, “Has a human being capacity?” He said: Yes, provided he possesses four characteristics - (he should be) free in respect of action1; in good health; complete in the possession of limbs, and in the possession of capacity given him by Allah. Now when all these qualities coexist, then the man is said to be capable (mustati).
He was asked, “For instance?” and the Imam said: (Suppose) there is a man who is free to act, in good health, possessing normal limbs. It is not possible for him to fornicate unless he sees a woman. Now when he meets the woman,2 it may either be that he is chaste, and prevents himself (from sin) as did Joseph, on whom be peace; or that he may act freely with her and fornicate and then he is a fornicator. He cannot be said to have obeyed Allah under compulsion (in the first case); nor can he be said to have disobeyed Him by being overpowered (in the second case).3
And Imam Ja'far as-Sadiq was asked concerning the saying of Allah the Mighty and Glorious:
“And they had been summoned to prostrate themselves while they were yet unhurt” (Qur'an 68:43).
He said: (That is) they were capable of acting as they were commanded and of abstaining from that which was prohibited, and for this reason they were tested.4 And Imam Abu Ja'far (Muhammad al-Baqir) on whom be peace, said: In the Torah it is written “O Moses (says Allah), verily I have created thee and chosen thee and guided thee and given thee strength and commanded thee to obey Me, and prohibited thee from disobeying Me.
Now if thou wilt obey Me, I shall help thee towards My obedience; but if thou wilt disobey Me, I shall not aid thee in thy disobedience to Me. It is for me to show kindness to thee in respect of thy obedience; and it is for Me to charge thee for thy disobedience to me”.5