Full Idea
It is implicit in the categorical imperative that I can will that everyone should adopt as a maxim only what everyone else can also will that everyone should adopt as a maxim.
Gist of Idea
I can only universalise a maxim if everyone else could also universalise it
Source
Thomas Nagel (Equality and Partiality [1991], Ch.5)
Book Reference
Nagel,Thomas: 'Equality and Partiality' [OUP 1995], p.48
A Reaction
This is a nice move, because it shifts the theory away from a highly individualistic Cartesian view of morality towards the idea that morality is a community activity.