Full Idea
The obligation to keep a promise does not derive from the institution of promising, ..but from the fact that in promising I freely and voluntarily create a reason for myself.
Gist of Idea
Promises hold because I give myself a reason, not because it is an institution
Source
John Searle (Rationality in Action [2001], Ch.6.IV)
Book Reference
Searle,John R.: 'Rationality in Action' [MIT 2001], p.198