Marx's theory of surplus value is another important part of his system. In half a day, let us say, the laborer can produce a value equivalent to the cost of his labor power; that is, he can produce in half a day enough to maintain himself for the whole day. But he sells to the capitalist his labor power for the whole day. The capitalist pays him the cost of the labor power, or in this instance, a half-day's product, and gets the other half-day's product for nothing. This exploitation of the laborer is the source and the only source of profits and rent, according to Marx. If Marx is correct in this, the interesting question arises why do capitalists sometimes prefer to invest their money in machinery and expensive plant where little labor is required rather than in those industries where much labor and little machinery are required? Or, to put it differently, how does it happen, if the exploitation of labor is the sole source of profit, that a person can get the same rate of return from an investment in a business where there is little labor to be exploited but where there is much machinery upon which interest must be paid, that he can get from a business where there is much labor to be exploited? The answer is that Marx was mistaken and that the exploitation of labor is evidently not the complete explanation of profits and interest.