Much can be learned from tracking Karl Marx’s use of literary texts in his footnotes, a practice that best can be understood in the context of his classical rhetorical training. His annotations, Dr. Engel argues, both contribute to and—as a kind of counter discourse—reflect the larger dialectical process carried out in his critique of political philosophy. Even though Marx is not writing a literary text as such, he is in fact doing a fair amount of literary criticism, all neatly tucked away in his notes, going so far as to quote long passages from key works in the classical tradition and from the English Renaissance that he then annotates.

"Marx's scholia: Annotations Involving Classical and English Literary Texts in Capital,"
 in Connotations: A Journal for Critical Debate 29  (2020), 190-219.