So long as we looked upon the production of value and the value of
 the product of capital individually, the bodily form of the commodities
 produced was wholly immaterial for the analysis, whether it was
 machines, for instance, corn, or looking glasses. It was always but a
 matter of illustration, and any branch of production could have served
 that purpose equally well. What we dealt with was the immediate
 process of production itself, which presents itself at every point as the
 process of some individual capital. So far as the reproduction of capital
 was concerned, it was sufficient to assume that that portion of the
 product in commodities which represents capital-value finds an
 opportunity in the sphere of circulation to reconvert itself into its
 elements of production and thus into its form of productive capital; just
 as it sufficed to assume that both the labourer and the capitalist find in
 the market those commodities on which they spend their wages and the
 surplus-value. This merely formal manner of presentation is no longer
 adequate in the study of the total social capital and of the value of its
 products. The reconversion of one portion of the value of the product into
 capital and the passing of another portion into the individual
 consumption of the capitalist as well as the working-class form a
 movement within the value of the product itself in which the result of the
 aggregate capital finds expression; and this movement is not only a
 replacement of value, but also a replacement in material and is therefore
 as much bound up with the relative proportions of the value-components
 of the total social product as with their use-value, their material shape.
 Simple reproduction, reproduction on the same scale, appears as an
 abstraction, inasmuch as on the one hand the absence of all
 accumulation or reproduction on an extended scale is a strange
 assumption in capitalist conditions, and on the other hand conditions of
 production do not remain exactly the same in different years (and this is
 assumed). The assumption is that a social capital of a given magnitude
 produces the same quantity of commodity-value this year as last, and
 supplies the same quantum of wants, although the forms of the
 commodities may change in the process of reproduction. However, as far
 as accumulation does take place, simple reproduction is always a part of
 it, and can therefore be studied by itself, and is an actual factor of
 accumulation. The value of the annual product may decrease, although
 the quantity of use-values may remain the same; or the value may
 remain the same although the quantity of the use-values may decrease;
 or the quantity of value and of the reproduced use-values may decrease
 simultaneously. All this amounts to reproduction taking place either
 under more favourable conditions than before or under more difficult
 ones, which may result in imperfect — defective — reproduction. All this
 can refer only to the quantitative aspect of the various elements of
 reproduction, not to the role which they play as reproducing capital or as
 a reproduced revenue in the entire process.