(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 your factory with no living labor, there could be a difference between the value of the output goods and the input materials/machines/whatever. The added value would not come from the machines, though. It would come from the labor from other factories that make a good with the same use-value. If society produced all of that good completely without labor input, then there would be no added exchange value when it is produced.