Neoclassical theoretical harmony was further disturbed by John Maynard Keynes (1883-1964), a Cambridge economist, admirer of Marshall, and member of the Bloomsbury circle of artists and intellectuals. During the 1920s Keynes began systematically to demolish the postulates of the prevailing neoclassical approach-for example, the neoclassical notion that unemployment was "voluntary." He increasingly came to think that economic systems did not automatically right themselves in reaching "the optimal level of production." But such was the domination of neoclassical theory over economic thought that Keynes's criticism would have been ignored were it not for the Great Depression-when markets proved incapable not only of optimization but also merely keeping workers employed. Keynes's The General Theory of Employment, Interest and Money (1936) found that the creation of demand by supply (as with Say's law) could occur at any level of employment or income, including very low levels of employment, so that full employment was but one of many economic possibilities for capitalism. The particular level of employment in an economy, Keynes thought, was determined by the aggregate demand for goods and services. Assuming that the government had a neutral effect, two groups influenced this total demand: consumers buying consumption goods and investors buying production equipment. Consumers increased spending as their incomes rose. But this was not the key to explaining the overall level of employment, for consumption depended on income, which depended on something else. In the Keynesian system, real investment (spending on new factories, tools, machines, and larger inventories of goods) was the crucial variable: changes in real investment fed into the other areas of an economy, expanding the whole economy. Investment resulted from decisions made by entrepreneurs under conditions of risk. Investment could be postponed. The decision to invest, Keynes said, depended on comparisons between the expected profits (from the investment) and the prevailing rate of interest. Here the crucial component was "expectation," or more generally the degree of investor confidence. The cost of the capital used in investment, the interest rate, Keynes explained in terms of speculation about future stock price, which in turn determined interest rates as savings moved from one fund to another. The decision to invest also depended essentially on expectations about the future. When investors bought machines, they provided income to machine builders (companies and employees), and these spent money, further increasing national income, with the "multiplier effect" (the degree of economic expansion induced by an initial investment) varying with the proportion of additional income that was spent rather that saved (the marginal propensity to consumer), and so on; a decrease in real investment had the reverse effects. The government could influence this process through interest rates and other monetary policies, shifting the economy from one equilibrium level to another, generally to higher employment levels (Moggridge 1980).