Keynesian economic theory, as distinct from Friedman and the Chicago School that led us down the rabbit hole. Nixon set the stage for rampant inflation, Carter couldn't stop it. Reagan deregulated, thus destabilized, lending and investment, Bush (es) and Clinton embraced Wall Street and neo-liberalism. Obama took over an economy ravaged by the sub-prime collapse, then four years of corruption. Biden inherited an unholy mess. He has also apparently read (and lived) quite a bit of economics and history.