marx actually addresses your argument in the first few pages of his book. he clarifies that taking longer than usual to do something does not impart more value to it. the labor he refers to in his theory is an abstraction. it is an idealized form of labor in the platonic sense, not referring to hours physically worked. in other words, what does it take to fabricate a given object under average conditions. that obviously varies based on the technological level of a society. creating a tool like a hammer or wrench, for example, is obviously easier with the equipment of a contemporary machine shop than a blacksmiths shed in the middle ages.
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