"Why does this strange phenomenon occur? What causes it? O Lord in your mercy give me light to see, for it may be that the answers to my question lies in the secret punishment of man and in the penitence which casts a deep shadow on the sons of Adam. Why does this strange phenomena occur? What causes it? The mind gives an order to the body and is at once obeyed, but when it gives an order to itself, is resisted. The mind commands the hand to move and is so readily obeyed that the order can scarcely be distinguished from its execution. Yet the mind is mind and the hand is part of the body. But when the mind commands the mind to make an act of will, these two are one and the same and yet the order is not obeyed. Why does this happen? What is the cause of it? The mind orders itself to make an act of will, and it would not give this order unless it willed to do so; yet it does not carry out its own command. But it does not fully will to do this thing and therefore its orders are not fully given. It gives the order only in so far as it wills, and in so far as it does not will the order is not carried out. For the will commands that an act of will should be made, and it gives this command to itself, not to some other will. The reason, then, why the command is not obeyed is that it is not given with the full will. For if the will were full, it would not command itself to be full, since it would be so already. It is therefore no strange phenomenon partly to will to do something and partly to will not to do it. It is a disease of the mind, which does not wholly rise to the heights where it is lifted by the truth, because it is weighed down by habit. So there are two wills in us, because neither by itself is the whole will, and each possesses what the other lacks."
-- from Augustine's Confessions