I think both Machery and Althusser tried to articulate and to clarify the position of literature in the ideology
or so-called ISA. However, the mechanism of the social activities is not so idealistic and imaginary; the form
of literature or literary works might include vaious and diverse layers of relations among the author/ individuals
and the society. From the implication of these relations, in the essay of "Categories for a Materialist Criticism," Terry Eagleton tries to clarify and to define the relations of literary productions and other modes of production in the society. In this essay, Eagleton explain some possible directions / connections between literature and society , of course, including the discussion of the ideological form. I think that Eagleton would like to orginize these complicate relations in systematic way. With the reference of Althusser and
Machery's concept, they establish systematic perspective of Literature in Marxism theory; furthermore, the discussion /analysis of Eagleton to Literature / Criticism as ideological form seems to follow up the previous conclusion with definitive analysis and explainations.