This article analyzes the determinants of liquidity crises based on the dynamics of banking and finance under Knightian uncertainty. In this perspective, the facts of the global financial crisis seem to confirm Minsky's hypothesis of endogenous financial instability derived from Keynes's theory of liquidity and expectations. Conventional expectations allow overcoming uncertainty via the liquidity of secondary markets and, in turn, of banks' liabilities that are accepted as money. However, the failure of existing conventions drives the system into uncertainty-driven liquidity spirals, which are the more dangerous the more private money financial intermediaries have managed to create in the first place. Despite limited availability of data that can proxy for Knightian uncertainty, this approach to liquidity problems may explain better than others how a relatively small shock, such as the default of U.S. subprime mortgages, could trigger a worldwide systemic crisis.