Where historical institutionalists have stressed the path-dependent efficiencies that stabilise policy orders, their rationalist assumptions have increasingly obscured the scope for instability. To redress such oversights, I integrate historical institutionalist insights regarding incremental change with discursive institutionalist analyses of interpretive tensions in a way that accords with Daniel Kahneman’s analyses of shifting ‘fast’/principled and ‘slow’/cognitive biases. The resulting framework posits that initial principled constructions of policy ideas are undermined where their subsequent ‘intellectual conversion’ limits flexibility and legitimacy. Empirically, I contrast the practices of George HW Bush and John Howard, as each broke anti-tax promises. Bush’s intellectual justifications undermined his credibility, but Howard’s principled justifications enabled his success. This analysis has implications for theories of institutional agency and dysfunction.