Say's law
While economists have abandoned Say's law as a true law that must always hold, most still consider Say's Law to be a useful rule of thumb which the economy will tend towards in the long run, so long as it is allowed to adjust to shocks such as financial crises without being exposed to any further such shocks. The applicability of Say's law in theoretical long-run conditions is one motivation behind the study of general equilibrium theory in economics, which studies economies in the context where Say's law holds true.