A basis for legitimate legal and political power in the idea of a contract. Contracts are things that create obligations, hence if we can view society as organized ‘as if’ a contract had been formed between the citizen and the sovereign power, this will ground the nature of the obligations of each to the other. This form of theory is prominent in Hobbes. In Locke and Rousseau the idea becomes one of a contract between citizens, as a result of which power is vested in government, rather than that of a contract between citizen and sovereign. This aspect is also reproduced in later contractarian writers such as Rawls. Social contract theory needs to explain whether the contract is thought of as having actually taken place, or as implied by social conformity, or as merely hypothetical, with the idea being that a legitimate body politic is one that a suitably placed agent could rationally have contracted into. Hume’s essay ‘Of the Original Contract’ is a devastating critique of some uses of the notion, primarily on the grounds that both the obligation we owe to contracts, and that which we owe to civil society, are constructions which themselves stand in need of some other fundamental basis, which he finds in the unforced growth of useful conventions. See also contractarianism, general will, original position, state of nature.

http://www.open.edu/openlearn/history-the-arts/culture/philosophy/what-the-social-contract An introduction to the social contract

http://www.econlib.org/library/LFBooks/Hume/hmMPL35.html Hume’s critique in ‘Of the Original Contract’