[T]he product as value must be the embodiment of social labour and, as such, be directly convertible from one use-value into all others. […] Thus the labour of individuals has to be directly represented as its opposite, social labour; this transformed labour is, as its immediate opposite, abstract, general labour, which is therefore represented in a general equivalent, only by its alienation does individual labour manifest itself as its opposite.  The commodity, however, must have this general expression before it is alienated.  This necessity to express individual labour as general labour is equivalent to the necessity of expressing a commodity as money.  The commodity receives this expression insofar as the money serves as a measure and expresses the value of the commodity in its price.  It is only through sale, through its real transformation into money, that the commodity acquires its adequate expression as exchange-value.  The first transformation is merely a theoretical process, the second is a real one.

Thus, in considering the existence of the commodity as money, it is not only necessary to emphasise that in money commodities acquire a definite measure of their value—since all commodities express their value in the use-value of the same commodity—but that they all become manifestations of social, abstract, general labour; and as such they all possess the same form, they all appear as the direct incarnation of social labour and as such they all act as social labour, that is to say, they can be directly exchanged for all other commodities in proportion to the size of their value; whereas in the hands of the people whose commodities have been transformed into money, they exist not as exchange-value in the form of a particular use-value, but as use-value (gold, for example) which merely represents exchange-value.  A commodity may be sold either below or above its value.  This is purely a matter of the magnitude of its value.  But whenever a commodity is sold, transformed into money, its exchange-value acquires an independent existence, separate from its use-value.  The commodity now exists only as a certain quantity of social labour-time, and it proves that it is such by being directly exchangeable for any commodity whatsoever and convertible (in proportion to its magnitude) into any use-value whatsoever.  This point must not be overlooked in relation to money any more than the formal transformation undergone by the labour a commodity contains as its element of value.  But an examination of money—of that absolute exchangeability which the commodity possesses as money, of its absolute effectiveness as exchange-value which has nothing to do with the magnitude of value—shows that it is not quantitatively, but qualitatively determined and that as a result of the very process through which the commodity itself passes, its exchange-value becomes independent, and is really represented as a separate aspect alongside its use-value as it is already nominally in its price.

“Within the value-relation and the value expression included in it, the abstractly general accounts not as a property of the concrete, sensibly real; but on the contrary the sensibly-concrete counts as the mere form of appearance or definite form of realisation of the abstractly general ... This inversion, by which the sensibly-concrete counts only as the form of appearance of the abstractly general and not, on the contrary, the abstractly general as property of the concrete, characterises the expression of value. At the same time, it makes understanding it difficult.” (The Value Form, p.39-140).

At another point Marx says:

“It is as if together with and besides lions, tigers, hares and all the other real animals, which as a group form the various genuses, species, subspecies, families etc of the animal kingdom, there also existed the Animal, the individual incarnation of the whole animal kingdom.” (Kapital 1st ed. p.234).

To decipher this statement by Marx, we must say that in commodity production the abstract universal really appears not as characteristic or attribute of the concrete, the sensuous-real (i.e. of the concrete modes of labour), since in order to abstract the specific universal features from these concrete modes of labour, it would need a unified organ, which does not exist in commodity production. The concrete kinds of labour are therefore not assimilated one with another through abstraction of some universal characteristics, but through comparison and equation of each of these kinds with a particular determined concrete kind which serves as phenomenal form of universal labour. In order that concrete labour becomes universal, universal labour must appear in the form of concrete labour […]