Actually, Buchanan’s theory supplanted an ideology — the faith in government as omniscient and benevolent. It replaced it with realism about the sociology of government and the logic of collective action. The theory’s explanatory and predictive power, Buchanan wrote, derives from its “presumption that persons do not readily become economic eunuchs as they shift from market to political participation.”

Concerning the cold logic of power maximization, Buchanan was as unsentimental as Machiavelli, whose “The Prince,” the primer on realism that announced political modernity, appeared exactly half a millennium ago, in 1513. Concerning the naturalness of self-interested behavior — its foundation in unchanging human nature — Buchanan stood in a line of thinkers that includes James Madison, the foremost realist among the cohort of realists we call the Founders. …

The political class is incorrigible because it is composed of — let us say the worst — human beings. They respond to incentives of self-interest. Their acquisitiveness is not for money but for the currency of power, which they act to retain and enlarge. This class can be constrained, if at all, not by exhorting them to become disinterested but by binding them with a constitutional amendment.