This doctrine is sometimes formally based upon the Marxian theory of value, and is sometimes defended independently of that theory. In the former case its groundwork is about as follows: By eliminating the factors of utility and scarcity, Marx found that the only element common to all commodities is labour, and then concluded that labour is the only possible explanation, creator, and determinant of value.[116] Since capital, that is, concrete capital, is a commodity, its value is likewise determined and created by labour. Since it cannot create value, for only labour has that power, it can contribute to the product of the productive process in which it is engaged only as much value as it originally received. Since it is only a reservoir of value, it cannot transfer more value than it holds and possesses. In the words of Marx, "the means of production transfer value to the new product, so far only as during the labour-process they lose value in the shape of the old use-value. The maximum loss of value that they can suffer in the process is plainly limited by the amount of the original value with which they came into the process, or, in other words, by the labour time necessary for their production. Therefore, the means of production can ever add more value to the product than they themselves possess independently of the process in which they assist. However useful a given kind of raw material, or a machine, or other means of production may be, though it may cost 150 pounds, or say 500 days' labour, yet it cannot, under any circumstances, add to the value of the product more than 150 pounds."[117] To view the matter from another angle: capital contributes to the product only sufficient value to pay for its own reproduction. When, as is the normal usage, the undertaker has deducted from the product sufficient value or money to replace the deteriorated or worn out machine, or other concrete capital, all the remaining value in the product is due specifically to labour. When, therefore, the capitalist goes further, and appropriates from the product interest and profits, he takes a part of the value that labour has created. He seizes the surplus value which labour has produced in excess of the wages that it receives. In ethical terms, he robs the labourers of a part of their product.