Socially necessary labour time in Marx's critique of political economy is what regulates the exchange value of commodities in trade and consequently constrains producers in their attempt to economise on labour. It doesn't 'guide' them, as it can only be determined after the event and is thus inaccessible to forward planning.
Unlike individual labour hours in the classical labour theory of value formulated by Adam Smith and David Ricardo, Marx's exchange value is conceived as a proportion (or 'aliquot part') of society's labour-time.
Marx did not define this concept in computationally rigorous terms, allowing for flexibility in using it in specific instances to relate average levels of labour productivity to social needs manifesting themselves as monetarily effective market demand for commodities. In addition, although it is axiomatic that socially necessary labour input determines commodity values, precise numerical calculation of such an input in relation to the value of a given commodity, i.e. the empirical regulation of the values of different types of commodities, is exceedingly difficult, due to incessantly shifting social, physical or technical circumstances affecting the labour process.
Read more about Socially Necessary Labour Time: Simplified Explanation of The Concept, Operation of The Law of Value, Marx and Ricardo, Criticism
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