HKB
absolutely good on the principle of autonomy (moral necessitation) is obligation. This, then, cannot be applied to a holy being. The objective necessity of actions from obligation is called duty.
From what has just been said, it is easy to see how it happens that, although the conception of duty implies subjection to the law, we yet ascribe a certain dignity and sublimity to the person who fulfils all his duties. There is not, indeed, any sublimity in him, so far as he is subject to the moral law; but inasmuch as in regard to that very law he is likewise a legislator, and on that account alone subject to it, he has sublimity. We have also shown above that neither fear nor inclination, but simply respect for the law, is the spring which can give actions a moral worth. Our own will, so far as we suppose it to act only under the condition that its maxims are potentially universal laws, this ideal will which is possible to us is the proper object of respect; and the dignity of humanity consists just in this capacity of being universally legislative, though with the condition that it is itself subject to this same legislation.
The Autonomy of the Will as the Supreme Principle of Morality
Autonomy of the will is that property of it by which it is a law to itself (independently of any property of the objects of volition). The principle of autonomy then is: "Always so to choose that the same volition shall comprehend the maxims of our choice as a universal law." We cannot prove that this practical rule is an imperative, i.e., that the will of every rational being is necessarily bound to it as a condition, by a mere analysis of the conceptions which occur in it, since it is a synthetical proposition; we must advance beyond the cognition of the objects to a critical examination of the subject, that is, of the pure practical reason, for this synthetic proposition which commands apodeictically must be capable of being cognized wholly a priori. This matter, however, does not belong to the present section. But that the principle of autonomy in question is the sole principle of morals can be readily shown by mere analysis of the conceptions of morality. For by this analysis we find that its principle must be a categorical imperative and that what this commands is neither more nor less than this very autonomy.
Heteronomy of the Will as the Source of all spurious Principles
of Morality