Labour is the measure of value. However, living labour in its exchange with capital has a lower value than materialised labour for which it is exchanged.
 Karl Marx, Das Kapital (1885). copy citation

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Author Karl Marx
Source Das Kapital
Topic value exchange
Date 1885
Language English
Reference
Note
Weblink https://www.marxists.org/archive/marx/works/download/pdf/Capital-Volume-...

Context

“The Ricardian school suffered shipwreck about the year 1830 on the rock of surplus-value. And what this school could not solve remained still more insoluble for its successor, Vulgar Economy. The two points which caused its failure were these:
1. Labour is the measure of value. However, living labour in its exchange with capital has a lower value than materialised labour for which it is exchanged. Wages, the value of a definite quantity of living labour, are always less than the value of the product begotten by this same quantity of living labour or in which this quantity is embodied. The question is indeed insoluble, if put in this form.” source