The Metaphysical Elements of Ethics by Immanuel Kant Chapter 17 Page 4

with its maxims adopted once for all, can never settle in a position of rest; but, if it is not rising, inevitably falls; because moral maxims cannot, like technical, be based on custom (for this belongs to the physical character of the determination of will); but even if the practice of them become a custom, the agent would thereby lose the freedom in the choice of his maxims, which freedom is the character of an action done from duty.