Full Idea
To say that someone 'ought' to do something is to imply that there is a reason for him to do it.
Gist of Idea
'Ought' implies that there is a reason to do something
Source
John Searle (Rationality in Action [2001], Ch.1.II)
Book Reference
Searle,John R.: 'Rationality in Action' [MIT 2001], p.29