But my comments below cut much deeper than complaints about the internally inconsistent and contradictory aspects of Marx’s work: they are a fundamental critique of the very conceptual coherence of Marx’s LTV and the idea of human SNLT.
In what follows, let us assume that the LTV is meant to be an empirical proposition and not some analytic statement (where it would true by definition, tautologous and empirically empty).
If the LTV is asserted as an empirical proposition, it seeks to explain empirically how the labour value in commodities is created.
I have already pointed to a fundamental problem identified by Piero Sraffa:
“There appears to be no objective difference between the labour of a wage earner and that of a slave; of a slave and of a horse; of a horse and of a machine, of a machine and of an element of nature (?this does not eat). It is a purely mystical conception that attributes to human labour a special gift of determining value. Does the capitalist entrepreneur, who is the real ‘subject’ of valuation and exchange, make a great difference whether he employs men or animals? Does the slave-owner?” (Sraffa, unpublished note, D3/12/9: 89, quoted in Kurz and Salvadori 2010: 199).The fact is, as Sraffa notes, labour power in production is not limited to human beings: historically and to this day, animal labour was, and still is in developing nations, a fundamentally important source of labour power.
if Marx’s abstract socially-necessary labour time (SNLT) units were meaningful measures of every type of heterogeneous, concrete human labour, then many types of the labour performed by animals ought to be measurable in terms of SNLT units too, especially when human labour can be substituted for animal labour (e.g., horse mills could be replaced with mills where humans do the work, ploughs can be pulled by humans, and so could carts or carriages).
Why does a coherent, empirically grounded theory of labour value exclude animals? This is exactly the point that Piero Sraffa is raising in his criticisms of the labour theory of value.
And there many other questions too.
Marxists might complain that animals do not sell their labour and are owned by humans and are a type of capital good. But so what?
We could easily imagine a counterfactual situation in which large numbers of people in industrial economies do not sell their labour, are enslaved and worked to death to produce commodities, and treated as capital goods. Would the commodities produced by slave labour cease to have a SNLT value? If slave labour does still have a SNLT value, then it follows that even if people receive no wages and are owned as slaves, then this doesn’t change the fact that they produce labour value in commodities.
But we can say the same thing about animals: animals receive no wages either, so why does their labour not count as SNLT?
One could go on to argue that animals can’t speak or spend money, but one could easily imagine counterfactual situations in which humans are made to work without spending money or or never taught natural human language. Would their labour cease to create SNLT?
Finally, it is no good for Marxists to fall back on an analytic definition of the LTV, where it is just true by definition. I have no reason to accept an analytic definition I think is incoherent and not empirically relevant.
We can see here how the LTV has severe conceptual problems from the very beginning.
Kurz, Heinz D. and Neri Salvadori. 2010. “Sraffa and the Labour Theory of Value: A Few Observations,” in John Vint et al. (eds.), Economic Theory and Economic Thought: Essays in Honour of Ian Steedman. Routledge, London and New York. 189–215.