How, then, can we define the Marxian symptom? Marx 'invented the symptom' (Lacan) by means of detecting a certain fissure, an asymmetry, a certain "pathological" imbalance which belies the universalism of the bourgeois 'rights and duties'. This imbalance, far from announcing the "imperfect realization" of these universal principles - that is, an insufficiency to be abolished by further development - functions as their constitutive moment: the 'symptom' is, strictly speaking, a particular element which subverts its own universal foundation, a species subverting its own genus. In this sense, we can say that the elementary Marxian procedure of 'criticism of ideology' is already 'symptomatic': it consists in detecting a point of breakdown heterogenous to a given ideological field and at the same time necessary for that field to achieve its closure, its accomplished form.- Slavoj Zizek, "The Sublime Object of Ideology"
This procedure thus implies a certain logic of exception: every ideological Universal - for example freedom, equality - is 'false' in so far as it necessarily includes a specific case which breaks its unity, lays open its falsity. Freedom, for example: a universal notion comprising a number of species (freedom of speech and press, freedom of consciousness, freedom of commerce, political freedom, and so on) but also, by means of a structural necessity, a specific freedom (that of the worker to sell freely his own labour on the market) which subverts the universal notion. That is to say, freedom is the very opposite of effective freedom: by selling his labour 'freely', the worker loses his freedom - the real content of this free act of sale is the workers enslavement to capital. The crucial point is, of course, that it is precisely this paradoxical freedom, the form of its opposite, which closes the circle of 'bourgeois freedoms'.
The same can also be shown for fair, equivalent exchange, this ideal of the market. When, in pre-capitalist society, the production of commodities has not yet attained universal character - that is, when it is still so-called 'natural production' which predominates - the proprietors of the means of production are still themselves producers (as a rule, at least): it is artisan production: the proprietors themselves work and sell their products on the market. At this stage of development there is no exploitation (in principle at least - that is, if we do not consider the exploitation of apprentices, and so on): the exchange on the market is equivalent, every commodity is paid its full value. But as soon as production for the market prevails in the economic edifice of a given society, this generalization is necessarily accompanied by the appearance of a new, paradoxical type of commodity: the labour force, the workers who are not proprietors of the means of production and who are consequently obliged to sell on the market their own labour instead of the products of their labour.
With this new commodity, the equivalent exchange becomes its own negation - the very form of exploitation, of appropriation of the surplus value. The crucial point not to be missed here is that this negation is strictly internal to equivalent exchange, not its simple violation: the labour force is not 'exploited' in the sense that its full value is not renumerated; in principle at least, the exchange between labour and capital is wholly equivalent and equitable. The catch is that the labour force is a peculiar commodity, the use of which - labour itself - produces a certain surplus over the value of the labour force itself which is appropriated by the capitalist.