Althusser is a structuralist Marxist. This should make you ask, how can that be? How can you combine Marxism, which relies on social/historical analysis, with structuralism, which relies on ahistorical/asocial analysis? Althusser answers that initially with distinction between ideologies (historical/social) and ideology (structural).
This essay as it appears in Critical Theory Since 1965 is an excerpt from a longer piece discussing the relation between the State and subjects. Althusser is asking why subjects are obedient, why people follow the laws, and why isn't there a revolt/revolution against capitalism. His view of ideology and ideologies comes out of his understanding of the relations between State and subject (between government and citizens), so it's worth while to examine those ideas for a minute.
The State, for Althusser, is the kind of governmental formation that arises with capitalism; a state (and you can substitute the word "nation" here to help conceptualize what the "State" is) is determined by the capitalist mode of production and formed to protect its interests. It is historically true (whether you are a Marxist or not) that the idea of nations as discrete units is coterminous with capitalism. It is also possible that democracy, as an ideology and/or a governmental form is also coterminous with capitalism, as democracy gives the "illusion" that all people are equal, and have equal power (and hence masks relations of economic exploitation).
Althusser mentions two major mechanisms for insuring that people within a State behave according to the rules of that State, even when it's not in their best interests (in regards to their class positions) to do so. The first is what Althusser calls the RSA, or Repressive State Apparatuses, that can enforce behavior directly, such as the police, and the criminal justice and prison system. Through these "apparatuses" the state has the power to force you physically to behave. More importantly for literary studies, however, are the second mechanism Althusser investigates, which he calls ISAs, or Ideological State Apparatuses. These are institutions which generate ideologies which we as individuals (and groups) then internalize, and act in accordance with. These ISAs include schools, religions, the family, legal systems, politics, arts, sports, etc. (as listed in the footnote on p.239). These organizations generate systems of ideas and values, which we as individuals believe (or don't believe); this is what Althusser examines. How do we come to internalize, to believe, the ideologies that these ISAs create, (and thus misrecognize or misrepresent ourselves as unalienated subjects in capitalism) Althusser's answer starts with the distinction between ideologies and ideology. IDEOLOGIES are specific, historical, and differing; we can talk about various ideologies, such as Christian ideology, democratic ideology, feminist ideology, Marxist ideology, etc. IDEOLOGY, however, is STRUCTURAL. Althusser says that ideology is a structure, and as such is "eternal," i.e. to be studied synchronically; this is why Althusser says (on p. 240) that ideology has no history. He derives this idea of ideology as a structure from the Marxist idea that ideology is part of the superstructure, but he links the structure of ideology to the idea of the unconscious, from Freud and from Lacan. Because ideology is a structure, its contents will vary, you can fill it up with anything, but its form, like the structure of the unconscious, is always the same. And ideology works "unconsciously." Like language, ideology is a structure/system which we inhabit, which speaks us, but which gives us the illusion that we're in charge, that we freely chose to believe the things we believe, and that we can find lots of reasons why we believe those things.
Althusser's first premise or thesis (p. 241a, in italics) is that "Ideology is a 'representation' of the Imaginary Relationship of Individuals to their Real conditions of existence." He begins his explanation of this pronouncement by looking at why people need this imaginary relation to real conditions of existence. Why not just understand the real?(p. 241b).
The first answer to this question, Althusser says, comes from the 18th century, and the idea that ideology comes from priests and despots. This is basically a conspiracy theory, which says that a handful of powerful men fooled the populace into believing these (falsified) representations/ideas about the world. (This is the eighteenth-century version of what I've said about feminism: that the men all got together one day and invented sexism, and the women were fooled by it).
The second (and, from the Marxist perspective, the correct) answer is that the material alienation of real conditions predisposes people to form representations which distance (alienate) them from these real conditions. In other words, the material relations of capitalist production are themselves alienating, but people can't quite deal with the harsh reality of this, so they make up stories about how the relations of production aren't so bad; these stories, or representations, then alienate them further from the real (alienating) conditions. The double distancing involved here, or the alienation of alienation, works like an analgesic, a pill, to keep us from feeling pain of alienation; if we didn't have these stories, we'd know the alienation of the real relations of production, and we'd probably revolt--or go nuts.
These ideas about representation and reality assume that what is reflected in the imaginary representation of the world found in ideology is the "real world," or real conditions of existence. Althusser says that ideology doesn't represent the real world per se, but human beings' RELATION to that real world, to their perceptions of the real conditions of existence. In fact, we probably can't know the real world directly; what we know are always representations of that world, or representations of our relation to that world. Ideology then is the imaginary version, the represented version, the stories we tell ourselves about our relation to the real world.
So the "real world" becomes, not something that is objectively out there, but something that is the product of our relations to it, and of the ideological representations we make of it--the stories we tell ourselves about what is real become what is real. That's how ideology operates.
In more Marxist terms, what ideology does is present people with representations of their relations to relations of production, rather than with representations of the relations of production themselves.
Marxism originally formulated ideology as an illusionary representation of the relations of people to real conditions. For example: my real condition, as a professor, is that of a "cultural worker," someone paid to perform intellectual labor in teaching. My salary is not nearly as large as that of a doctor, lawyer, movie star, or athlete (not even in minor league baseball!!). What might be considered my "exploitation," or my "real" economic conditions, are "masked" with an ideology--that teaching and being a college professor is of high moral/social value, if not of high economic value, that the rewards of teaching are immaterial, that I get social status and respect (instead of money) for being the repository of knowledge, etc. That's one notion of ideology: it keeps me happy, thinking that I am really an important person, when the real conditions of my economic existence show how relatively unimportant I am. I buy into that ideology (that being a professor is important), and am therefore willing to tolerate my exploitation (and my alienation from the products of my own mental labor, i.e. the surplus intellectual value I create in you) by believing that I get "other" rewards besides money for doing this job.
Althusser says, by contrast, that my ideology is an illusion, but it's an illusion, or an imaginary understanding, not of the relations of production themselves, but of my relation to them. Thus I think I'm cool because I'm not working in a factory, and I think I'm smarter than factory workers because I assume that factory workers aren't very bright, or they wouldn't be working in factories. The relations of production here are in assuming that factory workers lack education (that relations of production have structured a relationship between job and education); my relation to that relation of production is to feel superior to it. That's what Althusser says is ideology. Althusser's Thesis II appears on p. 242b: "Ideology has a material existence." It's important for Marxists always to be grounding their analysis in material practices, material relations (since Marxism is, after all, grounded in dialectical materialism)--so if we want to talk about IDEAS, we need to be able to talk about them as MATERIAL (so that we don't lapse into idealism, or an argument that ideas are more "real" than material objects). So, what Althusser does to assert that ideology is material is to say that ideology always exists in two places--in an apparatus or practice (such as a ritual, or other forms of behavior dictated by the specific ideology) and in a subject, in a person--who is, by definition, material. Note the insistence on the material in the italicized quote on 243.
On p. 244, Althusser says that ideology, as material practice, depends on the notion of the subject. Hence the two theses on 244: "there is no practice except by and in an ideology" and "there is no ideology except by the subject and for subjects". In short, there are no belief systems, and no practices determined by those belief systems, unless there is someone believing in them and acting on those beliefs.
Hence the final part of Althusser's argument: How is it that individual subjects are constituted in ideological structures? Or, in other words, how does ideology create a notion of self or subject?
All ideology has the function of constituting concrete individuals as subjects--of enlisting them in any belief system, according to Althusser. That's the main thing ideology as structure and ideologies as specific belief systems do--get people (subjects) to believe in them. There are three main points that Althusser makes about this process of becoming subjects-in-ideology.
1. We are born into subject-hood--if only because we're named before we're born; hence we're always-already subjects.
2. We are always-already subjects in ideology, in specific ideologies, which we inhabit, and which we recognize only as truth or obviousness. Everybody else's beliefs are recognizable as ideological, i.e. imaginary/illusory, whereas ours are simply true. Think, for example, about different religious beliefs. Everybody who believes in their religion thinks their religion is true, and everyone else's is just illusion, or ideology.
3. How does ideology (as structure) get us to become subjects, and hence not to recognize our subject positions within any particular ideological formation? How do we come to believe that our beliefs are simply true, not relative? Althusser answers this on 245b with the notion of INTERPELLATION. Ideology INTERPELLATES individuals as subjects. The word "interpellation" comes from the same root as the word "appellation," which means a name; it's not the same as the mathematical idea of "interpolation." Interpellation is a hailing, according to Althusser. A particular ideology says, in effect, HEY YOU--and we respond ME? You mean me?? And the ideology says, yes, I mean you.
You can see examples of this every day in commercials. I saw one the other night for a home gym system, claiming that "this machine will give you the kind of workout you desire, meeting your needs better than any other home gym." Each instance of "you" in that ad was an interpellation--the ad seeming to address ME PERSONALLY (in order to get me to see myself as the "you" being addressed, and hence to become a subject within its little ideological structure). This is also what Mr. Rogers does, when he looks sincerely into the camera and says "yes, I mean you." It also happens in the Uncle Sam recruiting posters which say "I want YOU for the Army."
Althusser makes some final points about ideology working this way to "hail" us as subjects, so that we think these ideas are individually addressed to us, and hence are true. He says that ideology, as structure, requires not only subject but Subject. In using the capital S, he invokes an idea similar to that of Lacan (whom Althusser studied and wrote about), that there is a small-s subject, the individual person, and a capital S Subject, which is the structural possibility of subjecthood (which individuals fill). The idea of subject and Subject also suggests the duality of being a subject, where one is both the subject OF language/ideology (as in being the subject of a sentence) and subject TO ideology, having to obey its rules/laws, and behave as that ideology dictates.
The interpellated subject in the ideology of the home gym commercial would thus order the gym, behave as if bodybuilding or rigorous exercise was a necessity, something of central importance. The Subject here would be some notion of physical perfection, or body cult, the rules that the subject is subjected to. Althusser uses the example of Christian religious ideology, with God as the ultimate Subject--the center of the system/structure.
On p. 248 Althusser links his ideas about ideology to Lacan directly, noting that the structure of ideology is specular (like Lacan's Imaginary, like the mirror stage).
There are a couple of things worth noting about Althusser as a "bricoleur" of other theorists. Althusser was enchanted by Freud, and even more enchanted by Lacan; the ideas of the imaginary, the mirror, the specular, and the subject/Subject are all gotten from or parallel to Lacanian notions. Also, as a Marxist, Althusser privileges SCIENCE as a form of knowing that is outside of any ideological structure, a type of knowledge that really IS simply true, because objective and material--hence his comment on 246 that the only way to know when ideology is ideological is through scientific knowledge.
Is this theory useful to literature? Yes, because it enables us to talk about how a literary text, as a subset or transformation or production of ideology (or of specific ideological formations) also constitutes us as subjects, and speaks to us directly. The most obvious form of how a literary text might interpellate us as subjects is one that uses direct address, when the text says "dear reader" (as Uncle Tom's Cabin does with annoying frequency). All texts interpellate readers by some mechanism, in some ways; all texts create subject positions for readers, whether that construction of subject positions is obvious or not. We will look at this idea of subject positions within literary texts further with Foucault.