In his 1788 essay "Federalist No. 51," James Madison defends the idea of a separation of the national government's powers through a system of checks and balances.
Madison argues that the executive, legislative and judicial branches of government will only function at the best of their ability if they are independent from one another. Yet if abuses or conflicts occur between them, they should be able to keep one another in check. This is the idea behind this famous quote from the essay: "ambition must be made to counteract ambition."