Sohn-Rethel: “Althusser defeats the purpose of his search for this question [implied but not formulated by Marx of which Capital is the answer] by insisting ‘que la production de la connaissance… constitue un processus qui se passe tout entier dans la pensee’. [Althusser] understands Marx on the commodity abstraction metaphorically, whereas it should be taken literally and its epistemological implications pursued so as to grasp how Marx’s method turns Hegel’s dialectic ‘right side up’.”
Zizek on Sohn-Rethel’s criticism of Althusser: “Sohn-Rethel is…quite justified in his criticism of Althusser, who conceives abstraction as a process taking place entirely in the domain of knowledge and refuses for that reason the category of ‘real abstraction’ as the expression of an ‘epistemological confusion’. The ‘real abstraction’ is unthinkable in the frame of the fundamental Althusserian epistemological distinction between the ‘real object’ and the ‘object of knowledge’ in so far as it introduces a third element which subverts the very field of this distinction: the form of the thought previous and external to the thought – in short: the symbolic order.”
Beyond Capital 