Independence was granted to central banks on a specific argument: that governments facing elections have an incentive to stimulate in the short run, and that removing the instrument from their hands solves a commitment problem. The argument is sound within its assumptions, and those assumptions describe a narrow policy space, in which the only decision is where to set a single rate and the only trade-off is between inflation and unemployment. Central banks now buy assets, direct credit through the sectors they choose to accept as collateral, and take decisions with visible distributional consequences. The delegation was granted for the narrow function and has been exercised over the wide one, without anyone having decided that it should be, and the justification has not been re-examined to match. Institutions rarely renegotiate a mandate that has grown in their own favour.