Before the 2008 crisis, the mainstream worldview among US macroeconomists was that economic fluctuations were regular and essentially self-correcting. In this column, IMF chief economist Olivier Blanchard explains how this benign view of fluctuations took hold in the profession, and what lessons have been learned since the crisis. He argues that macroeconomic policy should aim to keep the economy away from ‘dark corners’, where it can malfunction badly.