In an economic downturn, Adam Smith would expect the "invisible hand of the market" to regulate the economy. The term "invisible hand" was coined by Adam Smith in his book "The Wealth of Nations." In it, he explains that free market automatically reaches its own equilibrium, with little to no government intervention.
John Maynard Keynes has a different approach to economic downturns. In the Keynesian theory, he believes that the economy does not self-regulate, and needs a governent interference in order to prevent or minimize economic downturn. According to Keynes, the main cause of economic downturns is insufficent aggregate demand. To reverse this, artificial demand must be created.