Which group of economists believed that economic downturns were self-correcting, that is the forces of supply and demand would naturally bring the economy back to equilibrium?

The Say's law that associates the demand and supply in an economy actually applies to masses and not to single goods and commodities. Classical economists believe that the commodities markets will also always be in equilibrium because of flexible prices. If the supply is high and there is insufficient demand for it, it is a provisional situation. The prices for the commodity in question, drop, to associate the demand and supply and bring the situation back to equilibrium. How does this work? Well, what would you do if you had a commodity that you needed to sell but weren't able to secure a buyer. You'd obviously decrease the prices step by step, in a trial and error manner and finally reach a price that might lure a buyer to buy. It is a similar case with the aggregate demand and supply, according to the classical theorists.