Marx-Engels Correspondence 1868
Source: Karl Marx and Frederick Engels, Selected Correspondence (Progress Publishers, Moscow, 1975). Scanned and prepared for the Marxist Internet Archive by Paul Flewers.
With regard to Dühring.  It is a great deal from this man that he almost positively receives the section on Primitive Accumulation. He is still young. As a follower of Carey,  he is in direct opposition to the free-traders. Added to this he is a university lecturer and therefore not grieved that Professor Roscher, who blocks the way for all of them, should get some kicks.  One thing in his appraisal has struck me very much. Namely, so long as the determination of value by working time is left ‘vague’, as it is with Ricardo, it does not make people shaky. But as soon as it is brought into exact connection with the working day and its variations, a very unpleasant new light dawns upon them. I believe that an additional reason for Dühring to review my book at all was malice against Roscher. His fear of being treated like Roscher is certainly very easily perceptible. It is strange that the fellow does not sense the three fundamentally new elements of the book:
1) That in contrast to all former political economy, which from the very outset treats the different fragments of surplus value with their fixed forms of rent, profit, and interest as already given, I first deal with the general form of surplus value, in which all these fragments are still undifferentiated – in solution, as it were.
2) That the economists, without exception, have missed the simple point that if the commodity has a double character – use value and exchange value – then the labour represented by the commodity must also have a two-fold character, while the mere analysis of labour as such, as in Smith, Ricardo, etc, is bound to come up everywhere against inexplicable problems. This is, in fact, the whole secret of the critical conception.
3) That for the first time wages are presented as an irrational manifestation of a relation concealed behind them, and that this is scrupulously demonstrated with regard to the two forms of wages – time rates and piece rates. (It was a help to me that similar formulae are often found in higher mathematics.)
And as for Dühring’s modest objections to the determination of value, he will be astonished to see in Volume 2 how little the determination of value ‘directly’ counts in bourgeois society. Indeed, no form of society can prevent the working time at the disposal of society from regulating production one way or another. So long, however, as this regulation is accomplished not by the direct and conscious control of society over its working time – which is possible only with common ownership – but by the movement of commodity prices, things remain as you have already quite aptly described them in the Deutsch-Französische Jahrbücher... 
1. Marx refers to Eugen Dühring’s review of the first volume of Capital. Eugen Dühring (1833-1921) – German philosopher and economist, representative of reactionary petty-bourgeois socialism, his philosophical views, an eclectic mixture of positivism, metaphysical materialism and idealism, supported by some German Social-Democrats, were criticised by Engels in his Anti-Dühring – Progress Publishers.
2. Henry Charles Carey (1793-1879) – American vulgar economist, author of reactionary theory of harmony of class interests in capitalist society – Progress Publishers.
3. A reference to the criticism of Roscher’s vulgar economic views which Marx gives in the first volume of Capital (see Karl Marx, Capital, Volume 1 (Moscow, 1972), pp 95, 157, 199, 209, 220, 251, 343, 576.) [Available on the MIA at < http://www.marxists.org/archive/marx/works/1867-c1/index.htm >.] Wilhelm Georg Friedrich Roscher (1817-1894) – German vulgar economist, founder of so-called historical school of political economy – Progress Publishers.
4. An allusion to Engels’ essay ‘Umrisse zu einer Kritik der National Ökonomie’ (‘Outlines of a Critique of Political Economy’), see Karl Marx, Economic and Philosophic Manuscripts of 1844: Appendix – Progress Publishers.