51. It is, to take another example, impossible for a sick man to do a strong man’s work; but in this case the reason of the impossibility is of a different kind, for the man is rendered unable, by sickness, to do what he is naturally capable of doing. In this case, then, the cause of the impossibility is sickness, and this kind of impossibility is different from the first, since the man is hindered by bodily weakness from the possibility of doing.