Socially necessary labour time in Marxist political economy is the source of all value. Unlike simple labour hours in the classical labour theory of value explored by Adam Smith and David Ricardo, the Marxian unit of value is conceived as a fraction (or 'aliquot part') of the social total, not as a straightforward unit of an individual's time.

The clearest definition of socially necessary labour time is the amount of labour time required by a worker of average skill and productivity, working with tools of the average productive potential, required to produce a given commodity. Thus if the average productivity is that of a skilled worker, and a skilled worker produces a commodity in one hour, and an unskilled worker produces a commodity in four hours; then, the unskilled worker will have only contributed one hour's worth of value in terms of socially necessary labour time. The excess hours worked by the unskilled worker do not produce social value.

Marxist value theory treats value as a social relation, not as a material practice (x amount of bodily activity) or object (x amount of a substance), or private experience (x amount of satisfaction). Value here is a relationship between people organised into a society. 'Socially necessary labour time' encapsulates this essential 'relatedness' of value - it is labour time assessed in relation to social imperatives, not merely labour time. Effort expended on private goals such as hobbies is not valued, for this reason.

This vital distinction is what demarcates Marxists from other political economists, but is often overlooked in accounts of Marx's value theory. Marx understood that the casual reader might mistakenly treat his category as interchangeable with its Ricardian predecessor, and in later editions and the Afterword to the Second German Edition implores readers to pay particular attention to the mediations between that old category and the one his own theory sought to establish.

The centrality of the Social

Marx's assumption of a unitary, society's-eye-view from which to survey value has proven too elusive for many critics. Marx took a swipe at early ones, in a letter to a colleague called Kugelmann, thus:

All that palaver about the necessity of proving the concept of value comes from complete ignorance both of the subject dealt with and of scientific method. . . Every child knows . . . that the masses of products corresponding to the different needs require different and quantitatively determined masses of the total labour of society. That this necessity of the distribution of social labour 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. . . And the form in which this proportional distribution of labour asserts itself, in a state of society where the interconnection of social labour is manifested in the private exchange of the individual products of labour, is precisely the exchange value of these products.

Despite such protestations, Marx has been criticised strongly for adding the socially-necessary qualification to labour time, notably by the libertarian philosopher Robert Nozick, in his influential Anarchy, State and Utopia, for whom it is an unjustified 'bolt-on' aspect of Marx's theory.