Full Idea
To make a promise is not merely to adapt an ingenious device for promoting the general well-being; it is to put oneself in a new relation to one person in particular, creating a specifically new duty to him, not reducible to promoting general well-being.
Gist of Idea
Promises create a new duty to a particular person; they aren't just a strategy to achieve well-being
Source
W. David Ross (The Right and the Good [1930], p.38), quoted by Will Kymlicka - Contemporary Political Philosophy (1st edn) 2.3.a
Book Reference
Kymlicka,Will: 'Contemporary Political Philosophy (1st edn)' [OUP 1992], p.23
A Reaction
Of course, a politician might make a promise to society as a whole, but even there Ross seems to be right. 'I'll do it' is not the same as 'I promise you all I'll do it', which is more personal.