(I should say I'm a :LIB: and am reading the abridged Julian Borchardt version of Capital, so maybe there's an excised chapter that explains this or maybe there's a later chapter that does - I'm currently reading Decade of the Rate of Profit from Part II, chapters 13-15)
It seems that he insists surplus value is directly related to variable capital (i.e. labour), but other than semantically defining it that way, is this necessarily so? From the capitalist perspective how is labour really any different from any other input?
For instance, if one has a theoretical FALSC-style factory with no labour, surely one could still add surplus value to goods?
i think first thing to note (cos its easy to forget) is that the ltv was only ever intended to be applicable to a capitalist mode of production with proletarianised labour, so in falgsc there is no reason it would necessarily apply
and im just gonna post an old comment i made on fairly much this same question, because its a question ive grappled with a lot as well:
the short easy answer is that you can swindle a person but you cant swindle a machine, which you pay for at its full value, but that doesnt fully answer the question.
people sneer at the "social construct" argument, but for me its the most compelling one. value here is a social relation between people within a particular economic system, its not a natural law baked into the fabric of the universe - this might seem like a trite point, but modern economics likes to try and obscure that (and honestly i think its a thought trap that marx sometimes falls into as well). youll note early in capital that marx explicitly emphasises that the ltv doesnt apply to a slave-labour mode of production, but only to one where people "freely" sell their labour-power on a labour market, and i think thats key. so why is proletarian labour valuable in a way that slave labour isnt? or animal labour or machine labour? the ltv will treat the cost of a slave, animal, or machine as essentially the same - the labour used to produce/raise them, and then the ongoing cost of providing inputs (food, fuel, maintenance, etc), and you can say aha! that seems awfully like the value of labour-power (especially for the 1860s proletarian). but the difference here is that the proletarian is someone inside the "social contract" of a society that is nominally for their benefit as well (even if they may be a second class citizen). they are the ones selling their labour-power at its socially-determined value and buying the fruits of production at their socially-determined value and they themselves form part of that determination. but more core than that, in a society for the benefit of full humans, the time and energy of those full humans is considered valuable in a way that the time of a machine or a slave is not.
the machine or working animal or slave exist purely for the benefit of others, not for the benefit of themselves. if theyre not doing something that benefits their owner (or undergoing maintenance/sleep to allow them to continue benefiting their owner), what is the point of their existence in societys eyes? in a slave society it is a nonsense to care about the labour-time of the slave - if they work 16 hours instead of 6, what is the loss (beyond possibly working them to death and losing your commodity)? and likewise the machine or labour animal, society does not put a value on their time as a thing in itself (only the additional fuel/maintenance costs).
but the free labourer acts for themselves, and their time has actual value for the other things that they could be doing instead. not least of which is to spend the time labouring to provide their own necessities as things to use for themselves, not as commodities. and in theory, if people were truly free and there were no restrictions, they could go out and till the land to provide for their own necessities and withdraw from the commodity system completely (which is exactly what happened in the settler colonies where land was "free" (*cough*) - it was hard to hire workers because everyone coming over would bugger off and start their own homestead instead, so proletarianisation had to be enforced with indentures and land costs etc). so their time working for an employer for money to buy commodities has to compete with time working for themselves to provide their own necessities, and also time for themselves for whatever sort of leisure - because that time is also socially valuable for a "free" labourer in a way that it isnt for a slave or a machine.
as "free" humans, we value our own time and effort, and society says that we are allowed to value them and expect a fair recompense - but theres just that inherent trick in there that is the core aspect of the capitalist system, where what it tells us is the fair recompense is just what we need to afford our living costs, and not actually related in any meaningful way to the amount of useful stuff we actually produce. and thats where you can swindle a "free" person but not a machine.
In addition to this, the upward pressure that proletarians apply to value is that they resist exploitation to some degree. Slaves, machines and animals do not push back against their exploitation.
Why is this relevant? Absent any proletarian resistance (in a FALGSC system or a slave society) the cost of a commodity would purely be the cost of inputs. There is no surplus value in a post-scarcity system. Everything is at-cost, even if they still used markets to organize production and distribution.
When proletarians demand a wage for their time, and later demand certain wage increases or benefits, this creates an increased cost to all producers in that system. A large cost that can be reduced to increase surplus value. The more ruthless and exploitative a producing employer is, the more comparative advantage they have over the other producing employers. Thus we can see how surplus value is linked inherently to the degree of exploitation of its proletarian workforce.
The flip side of course is that proletarians want high wages and low commodity prices, a contradictory arrangement. The also enforce this downward force on their own wages when they seek a cheap commodity. The downward force on their wages drives them to further seek cheap commodities. It’s a feedback loop and this is why the rate of profit falls. The entire system enforces a downward force on proletarian wages, and proletarian wages are where profits (surplus value) arises from.
This is one of the inherent contradictions of the capitalist system