The economics of Kalecki was based, more explicitly and systematically than that of Keynes, on the principle of the circular flow of income that goes back to the Physiocrat François Quesnay. According to that principle, income is determined by expenditure decisions, not by the exchange of resources (capital or labor). Kalecki and Keynes claimed that in capitalist economy, production and employment levels (economic equilibrium) are determined foremost by the magnitude of investment by business enterprises (the crucial "driver of the business cycle"), not by price and wage flexibility. Savings are determined by investments, not the other way around. Contrary to the Ricardian, Marxian and Neoclassical economics, Kalecki asserted that higher wages lead to fuller employment. His monetary theory was rooted in the business cycle theory of Knut Wicksell. Quesnay's circular flow of income fell into disrepute in the political economy of the 19th century, when the idea that prices integrate exchange decisions gained ground, but was revived by Joseph Schumpeter, who pointed out the necessity of considering the circular flow of income (recognition of the economic cycle) as an integrating factor in order to gain a comprehensive understanding of the total economic process in a given period. The principle was discarded again with the arrival of neoliberal domination in economics and its main current defined by prices of economic equilibrium. Economist Jan Toporowski said that Kalecki's theory of the business cycle remains "the most serious challenge to general equilibrium macroeconomics", which has prevailed since the late 19th century. More than Keynes, Kalecki was skeptical about government's ability to sustain fiscal and monetary stimulus policies or of business support for full employment.