We are not master of anything along with Allāh, and we are not master of anything save what He makes us master of. So, when He makes us master of anything of which He is a superior Master over us He also assigns some duties to us; and when He takes it away He will take away those duties as well. 
 What Amīr al-mu’minīn means is that man does not enjoy regular mastery over anything, but such mastery as he is assigned by Allāh, and so long as this mastery lasts the obligations of the sharī‘ah also continue, whereas when the mastery is taken away the obligations too are lifted off, since in such a case the laying of obligations would mean placing of responsibility beyond capacity which cannot be allowed by any wise or prudent person. This is why Allāh has placed the responsibility of performing various acts after having conferred the necessary energy in the limbs. It follows that this responsibility would remain only so long as the energy subsists and that on the disappearance of the energy the responsibility for action would also disappear. For example, the obligation to pay zakāt (tax) applies only when there is wealth; but when Allāh would take away the wealth He would lift off the liability to pay zakāt because in such a case the laying of obligation is against prudence.