Full Idea
The act of association is a reciprocal commitment of public and private individuals, and each individual, contracting with himself, is under a twofold commitment, as a member of the sovereign to individuals, and as a member of the state to the sovereign.
Clarification
The 'sovereign' is the legitimate state power
Gist of Idea
The act of association commits citizens to the state, and the state to its citizens
Source
Jean-Jacques Rousseau (The Social Contract (tr Cress) [1762], I.7)
Book Reference
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.149
A Reaction
This seems to be expressed in modern terms as a mutual entailment of rights and duties. Where the traditional social contract is just between individuals, this seems to be a contract with a unified abstraction, of state commitment to citizens.