Some thoughts on the relationship between Marx's theory of value and his theory of exploitation--and why an increasingly popular alternate reading of what he's up to in Capital doesn't quite add up.
Marx clearly thinks that labour time used in the production of commodities for the market either is or creates value and he clearly thinks that this was true before capitalism (a system dominated by the employment of legally-free wage labourers for a profit). Indeed, under capitalism, there is a systematic divergence between value and price because capital is allocated to where it seeks the most profit, which for Marx is only the same as surplus value if the organic composition of capital is constant across industries.
In fairness to Roberts, Marx may not think that labour expended in production for immediate use (whether in the family or in a medieval manor) is or creates value. That is a subject of reasonable exegetical controversy. Caroligininan France had production for exchange, but a lot of production was for immediate consumption within the manor.
A society characterized entirely by simple commodity production (or by worker-owned firms interacting in a market) wouldn’t have exploitation. But it would have commodity festishism. Marx clearly thinks that a society of the associated producers would involve conscious deliberation over production and so he wouldn’t be happy with market forces even if there were no exploiting class. The root of the problem isn’t the consumption of the capitalist but the domination of social life by the M-C-M’ cycle.
For an analytical Marxist I think there should also be some kind of meta-question about whether "value" represents some sort of natural kind with an objective essence, and we're just trying to figure out the properties of this objective thing, or if its more like a category that's a human convention defined the way it is for pragmatic reasons, like being part of a useful simplified model (akin to those used in neoclassical economics, like 'perfect competition') or defined with the intent to direct our attention to conceptual distinctions we find significant (like how astronomers recently redefined the term 'planet' to distinguish between bodies that were just in the upper size range of 'belts' of objects like the asteroid belt and Kuiper belt vs. more isolated large bodies). Marx sometimes seems to talk in an essentialist way that suggests the former kind of interpretation, but in places he seems to equate value with the "natural price" things would have in a hypothetical scenario where supply and demand had reached a perfect equilibrium, which could be compatible with the latter interpretation.
My hunch is that this ambiguity has something to do with the kind of archaic state of philosophy of science in the 19th century. Nowadays everyone basically takes for granted some kind of "hypothetico-deductive" picture where scientists can use any methods they like to come up with candidate hypotheses which they then have to use to deduce predictions which can be compared to reality. But from Francis Bacon up until the early 20th century most everyone had a different picture where reality contains various objective essences and the human mind has a special ability called "induction" which in some straightforward way can figure out the essential features of some phenomenon based on empirical data and conceptual reasoning, without forming any conjectural hypotheses in advance (this model of science is why Newton claimed in the Principia that 'I frame no hypotheses'). Marx at times seems to be thinking in terms of a Baconian model when arguing for equating "value" with labor, like chapter 1 of Capital vol. 1 where he just assumes this thing called "value" must have some common denominator and labor is the only reasonable candidate, or in his 1868 letter to Ludwig Kugelmann at https://marxists.architexturez.net/archive/marx/works/1868/letters/68_07_11.htm where he writes:
"The chatter about the need to prove the concept of value arises only from complete ignorance both of the subject under discussion and of the method of science. Every child knows that any nation that stopped working, not for a year, but let us say, just for a few weeks, would perish. ... What is reasonable and necessary by nature asserts itself only as a blindly operating average. The vulgar economist thinks he has made a great discovery when, faced with the disclosure of the intrinsic interconnection, he insists that things look different in appearance. In fact, he prides himself in his clinging to appearances and believing them to be the ultimate. Why then have science at all?"
Marx clearly thinks that labour time used in the production of commodities for the market either is or creates value and he clearly thinks that this was true before capitalism (a system dominated by the employment of legally-free wage labourers for a profit). Indeed, under capitalism, there is a systematic divergence between value and price because capital is allocated to where it seeks the most profit, which for Marx is only the same as surplus value if the organic composition of capital is constant across industries.
In fairness to Roberts, Marx may not think that labour expended in production for immediate use (whether in the family or in a medieval manor) is or creates value. That is a subject of reasonable exegetical controversy. Caroligininan France had production for exchange, but a lot of production was for immediate consumption within the manor.
A society characterized entirely by simple commodity production (or by worker-owned firms interacting in a market) wouldn’t have exploitation. But it would have commodity festishism. Marx clearly thinks that a society of the associated producers would involve conscious deliberation over production and so he wouldn’t be happy with market forces even if there were no exploiting class. The root of the problem isn’t the consumption of the capitalist but the domination of social life by the M-C-M’ cycle.
For an analytical Marxist I think there should also be some kind of meta-question about whether "value" represents some sort of natural kind with an objective essence, and we're just trying to figure out the properties of this objective thing, or if its more like a category that's a human convention defined the way it is for pragmatic reasons, like being part of a useful simplified model (akin to those used in neoclassical economics, like 'perfect competition') or defined with the intent to direct our attention to conceptual distinctions we find significant (like how astronomers recently redefined the term 'planet' to distinguish between bodies that were just in the upper size range of 'belts' of objects like the asteroid belt and Kuiper belt vs. more isolated large bodies). Marx sometimes seems to talk in an essentialist way that suggests the former kind of interpretation, but in places he seems to equate value with the "natural price" things would have in a hypothetical scenario where supply and demand had reached a perfect equilibrium, which could be compatible with the latter interpretation.
My hunch is that this ambiguity has something to do with the kind of archaic state of philosophy of science in the 19th century. Nowadays everyone basically takes for granted some kind of "hypothetico-deductive" picture where scientists can use any methods they like to come up with candidate hypotheses which they then have to use to deduce predictions which can be compared to reality. But from Francis Bacon up until the early 20th century most everyone had a different picture where reality contains various objective essences and the human mind has a special ability called "induction" which in some straightforward way can figure out the essential features of some phenomenon based on empirical data and conceptual reasoning, without forming any conjectural hypotheses in advance (this model of science is why Newton claimed in the Principia that 'I frame no hypotheses'). Marx at times seems to be thinking in terms of a Baconian model when arguing for equating "value" with labor, like chapter 1 of Capital vol. 1 where he just assumes this thing called "value" must have some common denominator and labor is the only reasonable candidate, or in his 1868 letter to Ludwig Kugelmann at https://marxists.architexturez.net/archive/marx/works/1868/letters/68_07_11.htm where he writes:
"The chatter about the need to prove the concept of value arises only from complete ignorance both of the subject under discussion and of the method of science. Every child knows that any nation that stopped working, not for a year, but let us say, just for a few weeks, would perish. ... What is reasonable and necessary by nature asserts itself only as a blindly operating average. The vulgar economist thinks he has made a great discovery when, faced with the disclosure of the intrinsic interconnection, he insists that things look different in appearance. In fact, he prides himself in his clinging to appearances and believing them to be the ultimate. Why then have science at all?"