in neoclassical economics, of "labour services". The distinction between labour and labour-power, according to Marx, helped to solved a problem that David Ricardo had failed to solve, i.e. explaining why the surplus value resulting from profit normally arises out of the process of production itself—rather than in the investment of capital (e.g. the advance of money-capital in the form of wages) in labour-power (acquired from labourers). While Marx's concept of labour power has been compared to that of human capital, Marx himself may have considered a concept such as "human capital" to be a reification, the purpose of which was to imply that workers were a kind of capitalist. For instance, in Capital Vol. 2, Marx states: As commodity Under capitalism, according to Marx, labour-power becomes a commodity – it is sold and bought on the market. A worker tries to sell his or her labour-power to an employer, in exchange for a wage or salary. If successful (the only alternative being unemployment), this exchange involves submitting to the authority of the capitalist for a specific period of time. During that time, the worker does actual labour, producing goods and services. The capitalist can then sell these and obtain surplus value; since the wages paid to the workers are lower than the value of the goods or services they produce for the capitalist. Labour power can also be sold by the worker on "own account", in which case he is self-employed, or it can be sold by an intermediary, such as a hiring agency. In principle a group of workers can also sell their labour-power as an independent contracting party. Some labour contracts are very complex, involving a number of different intermediaries. Normally, the worker is legally the owner of his labour power, and can sell it freely according to his own wishes. However, most often the trade in labour power is regulated by legislation, and the sale may not be truly "free"—it may be a forced sale for one reason or another, and indeed it may be bought and sold against the real wishes of the worker even although he owns his own labour power. Various gradations of freedom and unfreedom are possible, and free wage labour can combine with slave labour or semi-slavery. The concept of labour power as a commodity was first explicitly stated by Friedrich Engels in The Principles of Communism (1847): Value Labour power is