Full Idea
The institution of promise keeping operates to provide portable reliability, by offering a formula that will confer high deliberative priority on what might not otherwise receive it.
Gist of Idea
Promise keeping increases reliability, by making deliberation focus on something which would be overlooked
Source
Bernard Williams (Ethics and the Limits of Philosophy [1985], Ch.10)
Book Reference
Williams,Bernard: 'Ethics and the Limits of Philosophy' [Fontana 1985], p.186
A Reaction
This is a bit pessimistic. We do not perceive promise keeping as a mere suggestion that we should bear something in mind when making a decision. 'May I rot in hell if I fail you'.