Marx’s “Socially Necessary Labour Time”: A Quick Overview and Critique

I have one question for Marxists. Where in Marx’s works does Marx give us a clear, explicit, detailed, and unambiguous definition of the crucial concept of “socially necessary labour time”?

I am not even sure Marx ever did this, but I will take this passage in a letter that Marx wrote to Louis Kugelmann as my source for what follows in this post:

“Every child knows a nation which ceased to work, I will not say for a year, but even for a few weeks, would perish. Every child knows, too, that the masses of products corresponding to the different needs required different and quantitatively determined masses of the total labor of society. That this necessity of the distribution of social labor in definite proportions cannot possibly be done away with by a particular form of social production but can only change the mode of its appearance, is self-evident. No natural laws can be done away with. What can change in historically different circumstances is only the form in which these laws assert themselves. And the form in which this proportional distribution of labor asserts itself, in the state of society where the interconnection of social labor is manifested in the private exchange of the individual products of labor, is precisely the exchange value of these products.”
Marx to Kugelmann, Letter, London, July 11, 1868.
https://www.marxists.org/archive/marx/works/1868/letters/68_07_11-abs.htm

Though Marx does not explicitly use the phrase “socially necessary labour time” it seems clear that he is referring to it here.

The first idea is that human labour is necessary for production and human survival, or at least for advanced industrial societies. That is true.

Then we have the crucial statement. The different types of goods that a capitalist economy produces require “different and quantitatively determined masses of the total labor of society.” So here labour is assumed to be an aggregate (admittedly of heterogeneous types of labour) and we can quantitatively measure how much of the aggregate is needed for the production of each type of good produced.

I assume Marx is thinking of labour time as the homogenous unit by which we measure the aggregate of the “total labor of society” and the parts therefore. If he is not, then what is he thinking of? Labour measured as the total number of workers?

I will continue to assume that labour time is the relevant concept. But immediately problems arise. If the car manufacturing sector takes 100,000 hours of total labour to produce 5000 cars, then is the value of an individual car 20 hours of labour time? What if this average conceals significant differences in various companies in terms of productivity and speed of the workforce? Is the average really justified as a measure of labour value?

And isn’t there a problem of aggregation of labour? If labour value is measured by labour time by means of the homogenous unit of hours worked, aren’t we faced with radically different types of labour value? Highly skilled labour (e.g., a professional surgeon) seems more valuable than unskilled manual labour (e.g., a person who mops floors). Does Marx’s labour theory of value make any allowances for this, or just regard one hour of doctor’s labour as equivalent to that of a janitor?

And, fundamentally, Marx makes it clear that he thinks his “social labour” proportions determine the exchange value (whether barter prices or money prices) of goods exchanged on the market:

“ And the form in which this proportional distribution of labor asserts itself, in the state of society where the interconnection of social labor is manifested in the private exchange of the individual products of labor, is precisely the exchange value of these products.”
Marx to Kugelmann, Letter, London, July 11, 1868.
https://www.marxists.org/archive/marx/works/1868/letters/68_07_11-abs.htm

So here we have the “transformation problem” right before our eyes. I can’t understand Marxists who tell me that there is no transformation problem in Marx’s work or that Marx never said that labour value determines exchange value or money prices. Clearly, he did.

But how on earth do Marxists solve the problem of how labour value as measured by “socially necessary labour time” determines prices on the market, and then empirically prove the values map to real world prices in real capitalist economies?

Actually, the Marxists have never solved these problems. And they never will.

The utter failure of the classical Marxist project is in the very assumption that “socially necessary labour time” determines exchange value or prices on the market. It does not.

This is an empirical fact. Most prices in modern economies are mark-up prices. It is not labour time per se but average unit cost of labour along with the average unit cost of all other non-labour factors at a given quantity of output that is used to calculate most prices. Given differing economies of scale, if you change the given quantity of output, then total average unit costs radically change, regardless of the total number of labour hours. After they calculate total average unit costs, businesses add a profit mark-up to this, nearly always. The profit mark-up can vary and is not related to labour.

There is, on straightforward empirical grounds, no rational reason to believe the Marxist mystical dogma that “socially necessary labour-time” determines exchange value or prices in modern capitalism.