Capital—the excess of assets over liabilities—determines solvency, so policymakers are used to thinking of it as a tool for keeping banks and the banking system safe. House Financial Services Chair Hensarling’s proposal to allow banks to opt for a simple capital standard that would substitute for other regulatory oversight is just the most recent example.

But bank capital also is a critical factor in the transmission of monetary policy. When central banks ease, their actions are intended to encourage banks to lend and firms to borrow.  And, to put it simply: healthy banks lend to healthy firms, while weak banks lend to weak firms (if at all)....