Full Idea
That duties are created irrespective of facts about character is obvious from the case of promises, which bind their makers irrespective of their motives or personality.
Gist of Idea
Promises create moral duties that have nothing to do with character
Source
Daniel Statman (Introduction to Virtue Ethics [1997], §5)
Book Reference
'Virtue Ethics', ed/tr. Statman,Daniel [Edinburgh 1997], p.22
A Reaction
Just occasionally a promise can be broken, by a sensitive and wise person. I promise to give your son some money; I then discover he is a drug dealer. Promises arise out of character, and cannot be made by robots.