Full Idea
In any 'ought' statement there is implicit a principle which says that the statement applies to all precisely similar situations.
Gist of Idea
An 'ought' statement implies universal application
Source
Richard M. Hare (Universal Prescriptivism [1991], p.456)
Book Reference
'A Companion to Ethics', ed/tr. Singer,Peter [Blackwell 1993], p.456
A Reaction
No two situations can ever be 'precisely' similar. Indeed, 'precisely similar' may be an oxymoron (at least for situations). Kantians presumably like this idea.