Chapter XII. Supplementary Remarks
There are just two causes that can change the price of production of a commodity.
First. A change in the general rate of profit. This can solely be due to a change in the average rate of surplus value, or, if the average rate of surplus value remains the same, to a change in the ratio of the sum of the appropriated surplus values to the sum of the advanced total social capital.
If the change in the rate of surplus value is not due to a depression of wages below normal, or their rise above normal — and movements ofthat kind are to be regarded merely as oscillations — it can only occur either through a rise, or fall, in the value of labour power, the one being just as impossible as the other unless there is a change in the productivity of the labour producing means of subsistence, i. e., in the value of commodities consumed by the labourer.
Or, through a change in the proportion of the sum of appropriated surplus values to the advanced total capital of society. Since the change in this case is not caused by the rate of surplus value, it must be caused by the total capital, or rather its constant part. The mass of this part, technically considered, increases or decreases in proportion to the quantity of labour power bought by the variable capital, and the mass of its value thus increases or decreases with the increase or decrease of its own mass. It also increases or decreases, therefore, proportionately to the mass of the value of the variable capital. If the same labour sets more constant capital in motion, it has become more productive. If the reverse, then less productive. Thus, there has been a change in the productivity of labour, and there must have occurred a change in the value of certain commodities.
The following law, then, applies to both cases: If the price of production of a commodity changes in consequence of a change in the general rate of profit, its own value may have remained unchanged. However, a change must have occurred in the value of other commodities.
Second. The general rate of profit remains unchanged. In this case the price of production of a commodity can change only if its own value has changed. This may be due to more, or less, labour being required to reproduce the commodity in question, either because of a change in the productivity of labour which produces this commodity in its final form, or of the labour which produces those commodities that go into its production. The price of production of cotton yarn may fall, either because raw cotton is produced cheaper than before, or because the labour of spinning has become more productive due to improved machinery.