One of Keen's most important arguments is about the relationship between private debt and economic growth. Because banks create money through lending, an increase in private borrowing directly increases the money supply and, with a time lag, economic spending. This means that rising private debt drives economic expansion. The dynamic works in reverse too: when private debt growth slows or reverses — when people and businesses pay down debt rather than taking on more — the money supply contracts, and economic spending falls. This is the mechanism Keen argues explains the Great Depression and the 2008 financial crisis: both were preceded by large buildups of private debt, and the contraction came when that debt burden became unsustainable and private borrowing slowed. For any builder: understanding that economic booms are partly debt-driven — and that debt-driven booms are structurally fragile — gives you a more accurate picture of why business environments shift suddenly and unexpectedly.