If I ordered a general to change himself into a sea-bird, and if the general did not obey, it would not be the general's fault. It would be my fault for ordering him to do the impossible.
One must demand of each and every one what he or she is capable of. Authority is first and foremost based on reason. If you order your people to throw themselves into the sea, you will have revolution on your hands. I have the rights to demand obedience because my orders are reasonable ones.
It is only with one's heart that one can see clearly. What essential is invisible to the eye.