Capital goods may be acquired with money or financial capital. In finance and accounting, capital generally refers to financial wealth, especially that used to start or maintain a business, sometimes referred to as Cash flow.
These distinctions of convenience carried over to neoclassical economics with little change in formal analysis for an extended period. There was the further clarification that capital is a stock. As such, its value can be estimated at a point in time, say December 31. By contrast, investment, as production to be added to the capital stock, is described as taking place over time ("per year"), thus a flow.
Earlier illustrations often described capital as physical items, such as tools, buildings, and vehicles that are used in the production process. Since at least the 1960s economists have increasingly focused on broader forms of capital. For example, investment in skills and education can be viewed as building up human capital or knowledge capital, and investments in intellectual property can be viewed as building up intellectual capital. These terms lead to certain questions and controversies discussed in those articles. Human development theory describes human capital as being composed of distinct social, imitative and creative elements:
Further classifications of capital that have been used in various theoretical or applied uses include:
In part as a result, separate literatures have developed to describe both natural capital and social capital. Such terms reflect a wide consensus that nature and society both function in such a similar manner as traditional industrial infrastructural capital, that it is entirely appropriate to refer to them as different types of capital in themselves. In particular, they can be used in the production of other goods, are not used up immediately in the process of production, and can be enhanced (if not created) by human effort.
There is also a literature of intellectual capital and intellectual property law. However, this increasingly distinguishes means of capital investment, and collection of potential rewards for patent, copyright (creative or individual capital), and trademark (social trust or social capital) instruments.
Karl Marx adds a distinction that is often confused with 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 produce consumer goods, and derived their value from them, being future goods.