Reserve Bank of Australia assistant governor Guy Debelle's speech on risk and uncertainty yesterday confirms two themes of recent RBA statements.The first theme is that rigid models are no substitute for informed judgment. Debelle argued that the risk models used by financiers over recent years cannot possibly remove all the inherent uncertainty that financial markets contain. That meant both financiers and regulators must continue to look outside their models and exercise judgment.The second theme is that a robust system requires occasional action against rising leverage. The RBA sees the recent global financial crisis largely as one more example of people borrowing too much. (Debelle specifically doubts whether the upsurge in derivatives made the crisis worse.) Argued Debelle: "Leverage played the major role in translating events which would have been somewhat damaging, but survivable, into events which were fatal."The message from Debelle is the message of Glenn Stevens' recent speeches. If the RBA judges that some part of the economy is overborrowing, it will actively discourage further borrowing.