Walter Wriston, Citicorp’s chief for nearly two decades until 1984, used to argue that banks’ didn’t need much, if any, capital. The global financial crisis put that view to rest. Today, we know that if banks are going to be able to absorb large unforeseen losses that would otherwise threaten financial stability, they need to finance themselves with equity, not just debt.

But how much capital do banks need to have to ensure the financial system is safe? Even after the financial crisis, answers to this question range widely, making it the single most contentious source of debate among bankers, regulators, and academics...