(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?
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