[Why is suicide held not to be right?] There is a doctrine uttered in secret that a man is a prisoner who has no right to open the door to his prison and run away; this is a great mystery which I do not understand. Yet I too, believe that the gods are our guardians, and that we are a possession of theirs.... And if one of your possessions, an ox or an ass, for example took the liberty of putting himself out of the way when you had given no intimation of your wish that he should die, would you not be angry with him, and would you not punish him if you could?... Then there may be reason in saying that a man should wait, and not take his own life until God summons him, as he is now summoning me.

Plato - Phaedo