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In 1873, Walter Bagehot, a prominent businessman in British high society and a journalist who served for 16 years as editor-in-chief of The Economist, wrote a treatise on banking and finance in which he left his most enduring mark on the world. In “Lombard Street: A Description of the Money Market,” he laid out a playbook for policymakers facing an unfolding economic and financial crisis. When up against such a challenge, Bagehot asserted, leaders must enact a policy response that is both swift and large. “By that policy,” he argued, “they allay the panic; by every other policy they intensify it.” ...