Full Idea
If all men were good, promising-breaking would not be good, but because they are bad and do not keep their promises to you, you likewise do not have to keep yours to them.
Gist of Idea
If men are good you should keep promises, but they aren't, so you needn't
Source
Niccolo Machiavelli (The Prince [1513], Ch.18)
Book Reference
Machiavelli,Niccolo: 'The Prince, selections from Discourses', ed/tr. Plamenatz,J [Fontana 1972], p.107
A Reaction
A rather depressing proposal to get your promise-breaking in first, based on the pessimistic view that people cannot be improved. The subsequent history of ethics in Europe showed Machiavelli to be wrong. Gentlemen began to keep their word.