One question that bothers many people who start to study Marxism is why labour power alone is regarded as the source of value in capitalism. Why not technology, energy flows or other forms of constant capital? What makes labour power special?
Human labour is said to be a unique commodity that produces more value than is required for its reproduction. To be sure, tools and natural resources can transfer value to the commodity, but that value stems from the labour that is socially necessary to produce them. Human workers, by contrast, contribute more value than they get back in the form of wages. This is the source of surplus-value.
But this argument is unsatisfactory since it doesn't explain why human labour alone produces value. At first glance, it seems obvious that robots and other machines are just as capable as humans of producing "more" than is needed for their reproduction. To rebuff this objection it isn't enough to point out that what robots produce "more" of isn't value, since that would make the argument circular. To avoid the charge of anthropocentrism, shouldn't one rather accept that the crucial input that creates value is simply energy, whether this passes through human bodies or not?
While this might be tempting, doing so would be a mistake. To explain why, let's try an (admittedly simplistic) thought experiment.* Say that we want to produce a fixed amount of a commodity. Superficially, it might seem that labour power and machinery are equivalent in the sense that both simply provide energy to the production process. Try, however, to increase either of them and see what happens. Introducing more or better machinery can be expected to increase productivity and prices will therefore tend to fall. In terms of the value law, the increasing energy flows will lower the amount of "socially necessary" labour for producing the commodity and hence its value. The opposite effect follows if we instead increase labour power. Assuming that the commodity will be sold and that wages remain constant, the increase in working-hours will drive up the costs for labour and hence also prices. In terms of the value law, the result is a more valuable commodity.
Why does the increase in the flow of energy results in a less valuable commodity in one case and a more valuable commodity in the other? If we want to avoid anthropocentrism, the only possible answer is that the form of wage labour is crucial, regardless of whether this labour power is provided by humans or non-humans. By the form of wage labour I mean that labour is treated as a commodity that is the private property of individual workers and sold for wages on a labour market. Using Marx's terms, we might say that wage labour alone is "abstract labour" capable of producing value, while other types of energy input are "concrete labour" - labour that is useful but fails to show up as value.
What matters, then, isn't whether labour power is human or not, but whether it is waged or not. The thought experiment above helps us see why. Due to the form of wage labour, increasing the input of labour power for a fixed amount of a commodity must result in increasing production costs, unless workers are found who are willing to work for cheaper wages. Due to the competition between capitals, increasing the production costs associated with labour will usually not be a viable strategy unless for some reason there is a general rise of the "socially necessary labour" for manufacturing it. Increasing other forms of energy input, by contrast, is usually done to lower production costs. The fact that energy has a unit cost - e.g. kilowatt hours of electricity - doesn't make it similar to labour power, since the overall effect will still be to cheapen production. Due to the competition between capitals, this will also result in a general decrease in the "socially necessary labour" needed for manufacturing the commodity. There is thus a crucial difference in regard to production costs, which means that labour power sets the baseline for value while other forms of energy subtract from value in proportion to their cheapness in relation to labour power.**
An interesting consequence - which might sound fanciful but which might well be spelled out more boldly than has been done so far - is that it is quite possible to imagine robots or animals producing value provided that these robots or animals are at the same time employed as wage-workers.
The crucial role of wage labour is shown by the fact that, just as waged robots or animals can produce value, human workers can lose their value-creating ability if their labour isn't provided as individual wage labour. Imagine, for example, a capitalist who pays a lump sum to a sub-contractor who agrees to provide the capitalist with all the necessary labour power. In such a case the number of workers can obviously be increased without adding to the value of the commodity. The same holds true for a labour force consisting of serfs producing for a capitalist market. The effect in both cases is similar to the effect of increasing productivity by un-waged machinery.***
In a nutshell, from the point of view of the value law, un-waged labour tends to behave like constant capital while waged machinery, if it were to exist, would tend to behave like variable capital.
That the form of wage-labour is crucial for the value law of course doesn't mean that individual capitalists must rely on wage-labour. While capitalism as a whole certainly depends on the production of surplus-value, individual capitalists compete for a share of the aggregate amount of surplus-value in various ways, above all by trying to increase productivity and cutting wage costs. This is why wage-labour can be central to capitalism while individual capitalists at the same time try to minimize their reliance on wage-labour. It is also why capitalism co-exists with a wide variety of un-waged forms of labour. Although the latter don't create value, they give individual capitalists a competitive edge against other capitalists and help them increase their share of the aggregate surplus-value produced in society.****
A final thought: does my suggestion that the labour law should be understood as non-anthropocentric mean that we might imagine a capitalism in which all human labour can be abolished and replaced by robots? Theoretically speaking, yes, although its hard to imagine such a society ever being realized. Furthermore, abolishing human labour would not mean the end of social conflicts. Indeed, such a society is likely be at least as conflict-ridden as today. To begin with, who says that robots must necessarily be docile (especially if they have evolved so far as to be able to replace human labour and to function as full-fledged free individuals owning their own labour power)? Secondly, the great mass of un-employed humanity would still need to be fed. Not all humans will be able to enter the capitalist class or remain there, and those who can't will hardly be docile either.
Footnotes:
* As mentioned, the thought experiment rests on many simplifying assumptions. I have regarded demand for the commodity as inflexible and simply assumed that it will be sold. I have also assumed that wages are inflexible (so that wage-labour isn't increased, for instance, by moving production to countries with cheaper wages). I have also disregarded the possibility that increasing the quantity of workers can have the effect of increasing productivity (e.g. though a division of labour). Finally, I have disregarded that technology too has costs that in some instances (e.g. in start-up phases) may off-set the tendency for technology to cheapen production.
** This relation is obscured by the fact of monopolistic price-setting, e.g. due to scarcity (see the chapters on ground rent in Capital, vol. III). Since almost all natural resources are to some extent defined by scarcity, "nature" is a realm where prices are almost never determined in accordance with the value law. See this old blog post for a discussion of how Marx treats scarcity.
*** Interestingly, it doesn't hold for most forms of slavery, since the slave-owner usually has to provide the costs for the reproduction of the labour force. This means that slavery approximates modern wage-labour better than serfdom (where the workers are to a greater extent responsible for their own subsistence).
**** This is why it is crucial to distinguish between what Marx calls the rate of profit (s/(c+v)) and the rate of surplus value (s/v). Natural resources (c) don't contribute to surplus value, but they do contribute to the rate of profit. Why? Because the rate of profit diminishes the costlier these resources are, just as it diminishes the costlier labour power (v) is. As Marx points out, the notion of the rate of profit is important since it is what most immediately motivates capitalists, namely the return on capital.
