But, by necessity, I do not mean compulsion; but (as they term it) the necessity of immutability, not of compulsion; that is, a man void of the Spirit of God, does not evil against his will as by violence, or as if he were taken by the neck and forced to it, in the same way as a thief or cut-throat is dragged to punishment against his will; but he does it spontaneously, and with a desirous willingness.
And this willingness and desire of doing evil he cannot, by his own power, leave off, restrain, or change; but it goes on still desiring and craving. And even if he should be compelled by force to do any thing outwardly to the contrary, yet the craving will within remains averse to, and rises in indignation against that which forces or resists it.
-- Martin Luther, De Servo Arbitrio