Sismondi first proposed this problem before him. Malthus is more famous for his earlier writings which tried to prove the opposite problem, a general over-consumption, as an inevitability to be lived with rather than solved. Keynesian Keynesian economics, and underconsumptionism before it, argue that fiscal stimulus in the form of government deficit spending can solve general gluts. This is a demand side theory, rather than the supply-side theory of classical economics; the fundamental ideas are that savings in a recession or depression causes the paradox of thrift (excess saving, or more pejoratively, "hoarding"), causing a deficit of effective demand, yielding a general glut. Keynes locates the cause in sticky wages and liquidity preference. Marxian Karl Marx's critique of Malthus started from a position of agreement. Marx's idea of capitalist production, however, is characterized by his concentration on the division of labor and his notion that goods are produced for sale and not for consumption or exchange. In other words, goods are produced simply for the intention of transforming output into money. The possibility of a lack of effective demand, therefore, is held only in the possibility that there might be a time lag between the sale of a commodity (the acquisition of money) and the purchase of another (its disbursement). This possibility, also originally crafted by Sismondi (1819), endorsed the idea that the circularity of transactions was not always complete and immediate. If money is held, Marx contended, even if for a little while, there is a breakdown in the exchange process and a general glut can occur. For Marx, since investment is part of aggregate demand, and the stimulus for investment is profitability, accumulation will continue unhindered as far as profitability is high. However, Marx saw that profitability had a tendency to fall, which would lead to a crisis in which insufficient investment generates an insufficiency of demand and a glut of markets. The crisis itself would operate to raise profitability, which would start a new period of accumulation. This would be the mechanism for crisis occurring repeatedly. Post-Keynesian Some Post-Keynesian economists see the cause of general gluts in the bursting of credit bubbles, particularly speculative bubbles. In this view, the cause of a general glut is the shift from private sector deficit spending to private sector savings, as in the debt-deflation hypothesis of Irving Fisher and the Financial Instability Hypothesis of Hyman Minsky, and locate the paradox of