Say's law
The whole of neoclassical equilibrium analysis implies that Say's law in the first place functioned to bring a market into this state: that is, Say's law is the mechanism through which markets equilibrate uniquely. Equilibrium analysis and its derivatives of optimization and efficiency in exchange live or die with Say's law. This is one of the major, fundamental points of contention between the neoclassical tradition, Keynes, and Marxians. Ultimately, from Say's law they deduced vastly different conclusions regarding the functioning of capitalist production.