Keynes overturns the classical presumption that markets naturally converge on full employment. His iconic proposition is that aggregate demand, shaped by expectations, investment and uncertainty, determines output and employment, allowing economies to settle into prolonged underperformance. The theoretical contribution is a macroeconomic ontology in which the whole cannot be reconstructed by merely aggregating individually rational decisions. Methodologically, Keynes develops conceptual variables—effective demand, liquidity preference, marginal efficiency of capital and the multiplier—to describe feedback among income, expenditure and expectation. The conceptual operation is systemic inversion: unemployment ceases to appear as temporary friction and becomes an endogenous possibility of monetary economies. The wider bridge reaches institutional economics, political theory, planning and economic geography. His treatment of uncertainty is especially decisive: future conditions cannot always be reduced to calculable risk, so conventions, confidence and public intervention shape economic reality. Markets are therefore governed ecologies of expectation rather than self-correcting mechanisms outside politics.