From Marginal Revolution:
Political writers have established it as a maxim, that, in contriving any system of government, and fixing the several checks and controls of the constitution, every man ought to be supposed a knave, and to have no other end, in all his actions, than private interest. By this interest we must govern him, and, by means of it, make him, notwithstanding his insatiable avarice and action, cooperate to public good…. It is, therefore, a just political maxim, that every man must be supposed a knave; though, at the same time, it appears somewhat strange, that a maxim should be true in politics which is false in fact.
David Hume, “Of the Independency of Parliament”, in Essays Moral, Political, and Literary. Rpt. Oxford: Oxford University Press, 1963 , pp. 40-47.