Full Idea
Kant's rule about universalisable maxims is useless without stipulations as to what shall count as a relevant description of an action with a view to constructing a maxim about it.
Gist of Idea
Universalising a maxim needs to first stipulate the right description for the action
Source
comment on Immanuel Kant (Groundwork of the Metaphysic of Morals [1785]) by G.E.M. Anscombe - Modern Moral Philosophy p.176
Book Reference
'The Is/Ought Question', ed/tr. Hudson,W.H. [Macmillan 1969], p.176
A Reaction
This is one of the key objections to Kant (along with his need for preliminary values). One man's 'terrorist' is another man's 'freedom fighter'. The charge adds up to Nietzsche's view, that Kant could never shake off his very conventional prejudices.