David Hume Quote
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. But to satisfy us on this head, we may consider, that men are generally more honest in their private than in their public capacity, and will go greater lengths to serve a party, than when their own private interest is alone concerned. Honour is a great check upon mankind: But where a considerable body of men act together, this check is, in a great measure, removed; since a man is sure to be approved of by his own party, for what promotes the common interest; and he soon learns to despise the clamours of adversaries.
Part I, Essay 6: Of The Independency of Parliament; first line often paraphrased as "It is a just political maxim, that every man must be supposed a knave." - Essays, Moral, Political, and Literary (1741-2; 1748)