In economics, physical capital or just 'capital' refers to a factor of production (or input into the process of production), such as machinery, buildings, or computers. The production function takes the general form Y=f(K, L), where Y is output, K is capital stock and L is labor. In economic theory, physical capital is one of the three primary factors of production, also known as inputsproduction function. The others are natural resources (including land), and labor — the stock of competences embodied in the labor force. "Physical" is used to distinguish physical capital from human capital (a result of investment in the human agent)) and financial capital. "Physical capital" refers to fixed capital, any kind of real physical asset that is not used up in the production of a product is distinguished from circulating capital.