Duty

For no phase of life, whether public or private, whether in business or in the home, whether one is working on what concerns oneself alone or dealing with another, can be without its moral duty; on the discharge of such duties depends all that is morally right, and on their neglect all that is morally wrong in life.

But which of you, having a servant ploughing or feeding cattle, will say to him, when he is come from the field: Immediately go. Sit down to meat.
And will not rather say to him: Make ready my supper and gird thyself and serve me, whilst I eat and drink; and afterwards thou shalt eat and drink?
Doth he thank that servant for doing the things which he commanded him?
I think not. So you also, when you shall have done all these things that are commanded you, say: We are unprofitable servants; we have done that which we ought to do.

After this, remember that you are a son. What does this character promise? To consider that everything which is the son’s belongs to the father, to obey him in all things, never to blame him to another, nor to say or do anything which does him injury, to yield to him in all things and give way, cooperating with him as far as you can. After this know that you are a brother also, and that to this character it is due to make concessions; to be easily persuaded, to speak good of your brother, never to claim in opposition to him any of the things which are independent of the will, but readily to give them up, that you may have the larger share in what is dependent on the will. For see what a thing it is, in place of a lettuce, if it should so happen, or a seat, to gain for yourself goodness of disposition. How great is the advantage. Next to this, if you are senator of any state, remember that you are a senator: if a youth, that you are a youth: if an old man, that you are an old man; for each of such names, if it comes to be examined, marks out the proper duties.

Obligation is the necessity of a free action, falling under a categorical imperative of reason.
An imperative is a practical rule, by which an action, in itself contingent, is rendered necessary, and differs in this point from a practical law, that whereas this last represents the necessity of an action, yet it does so irrespective of the consideration that such action may, of inward necessity, belong to an agent (e.g., a holy one), and yet, in the case of man, be merely fortuitous; for, where the action is already necessary, there no imperative can be expressed. An imperative is therefore a rule making necessary a subjectively contingent action, and thereby representing the subject affected by it as one who must necessitate his actions to harmonize with the rule. The categorical (i.e., absolute or unconditional) imperative is not one which commands mediately, or by the representation of any ulterior end whitherward the action might point, but is one which, by the bare representation of the act, cogitates it as immediately-incumbent, and makes it objectively-necessary. Imperatives of this sort, no practical doctrine, which treats of obligations, save Ethic singly, can present. All other imperatives are technical and conditioned.