Within classical economics, Adam Smith (Wealth of Nations, Book II, Chapter 1) distinguished fixed capital from circulating capital, including raw materials and intermediate products. For an enterprise, both were kinds of capital. Karl Marx adds a distinction that is often confused with David Ricardo's. In Marxian theory, variable capital refers to a capitalist's investment in labor-power, seen as the only source of surplus-value. It is called "variable" since the amount of value it can produce varies from the amount it consumes, i.e., it creates new value. On the other hand, constant capital refers to investment in non-human factors of production, such as plant and machinery, which Marx takes to contribute only its own replacement value to the commodities it is used to produce. It is constant, in that the amount of value committed in the original investment, and the amount retrieved in the form of commodities produced, remains constant. Investment or capital accumulation in classical economic theory is the production of increased capital. In order to invest, goods must be produced which are not to be immediately consumed, but instead used to produce other goods as a means of production. Investment is closely related to saving, though it is not the same. As Keynes pointed out, saving involves not spending all of one's income on current goods or services, while investment refers to spending on a specific type of goods, i.e., capital goods. The Austrian economist Eugen von Böhm-Bawerk maintained that capital intensity was measured by the roundaboutness of production processes. Since capital is defined by him as being goods of higher-order, or goods used to