Full Idea
In all social contracts, we find a union of many individuals for some common end which they all share. But a union as an end in itself which they all ought to share …is only found in a society insofar as it constitutes a civil state i.e. a commonwealth.
Gist of Idea
Personal contracts are for some end, but a civil state contract involves a duty to share
Source
Immanuel Kant (True in Theory, but not in Practice [1792], 2 Intro)
A Reaction
This makes a nice link between the contractarian individual morality of Hobbes and his social contract view of society. Kant seems to reject the first but accept the second. Presumably because the first implies benefit and the second implies duty.
Book Reference
Kant,Immanuel: 'Political Writings', ed/tr. Reiss,Hans [CUP 1996], p.73