Herewith I am returning the manuscripts  registered. As far as economics is concerned your article on Rodbertus  is very good. What I object to again is apodictic assertions in fields where you do not feel yourself sure and where you have exposed your weak spots to Schramm who has been skilled enough to nail them. This refers particularly to the abstraction which you have certainly run down much too much in general. In this case the difference is as follows: Marx summarises the actual content common to things and relations and reduces it to its general logical expression. His abstraction therefore only reflects, in rational form, the content already existing in the things. Rodbertus on the contrary invents a more or less imperfect logical expression and measures things by this conception to which the things must conform. He is seeking a true, eternal content of things and of social relations whose content however is essentially transient. Hence true capital. This is not present-day capital, which is only an imperfect manifestation of the concept. Instead of deducing the concept capital from the present, the only really existing capital, he has recourse to isolated man in order to arrive from present-day capital at true capital and asks what could function as capital in the productive process of such a man. Of course, simple means of production. Thus true capital is lumped together unceremoniously with the means of production, which depending on circumstances may or may not be capital. Thereby all bad properties, that is, all real properties of capital are eliminated from capital. Now he can demand that real capital should conform to this concept, that is, it should function only as simple social means of production, should discard everything that makes it capital and still remain capital and even just on that account become true capital...
Letters: Marx-Engels Correspondence 1884