These influential theories of the second half of the twentieth century, all of which are focused on language, have their origins in the linguistic theory of Ferdinand de Saussure (1857-1913), particularly his Cours de linguistique générale (1916) or Course in General Linguistics, taken from his students' lecture notes and published posthumously. Contrary to many of the linguistic theories of the day, which focused on diachronic linguistics or the changes in languages over time, Saussure developed a theory of synchronic language, how language works in the present. He argued that the relationship between the spoken word (signifier) and object (signified) is arbitrary and that meaning comes through the relationship between signs, which are for Saussure the union of signified and signifier. So the word "tree" means  by custom only and not through any intrinsic relationship between the sound and the thing. That's why both "arbol" and "tree" can both signify the same signified. English speakers construct meaning by distinguishing between tree and treat and trek as well as between tree and bush and flower. Meaning, then, comes from understanding what a thing IS NOT rather than from knowing in any kind of ontological sense what a thing IS. Meaning is constructed through difference, particularly through binary pairs (man/woman, good/evil). There is no absolute Platonic ideal "out there" to anchor meaning. There is no truth that is not constructed. There is nothing outside language. Language speaks (through) us. Language is thus a system of signs or a semiotic system, but merely one of many, all of which construct meaning, which does not exist outside the semiotic system.

Some anthropologists seized on Saussure's theory of semiotic structure to analyze and understand a variety of cultures, which, they theorized, could be mapped "scientifically" through a structuralist methodology. Literary critics also drew on Structuralism to map the semiotics of genres and individual works and, in the process, to challenge the formalist / humanist criticism that had dominated literary study in the first half of the century. Perhaps most influential was Roland Barthes (1915-1980) who proclaimed the death of the author. That is, if language speaks us, then the author is relatively unimportant to the process of writing.

Jacques Derrida (1930- ) used Saussure's insights to develop Deconstruction, a perspective that focuses on the lack of a truth "out there" or at the center to provide meaning. He showed how all Western philosophical systems are dependent on a center (God, the self, the unconscious). But structuralism had shown that the center is a fiction, merely another signified that has no being beyond language. Furthermore, Derrida focused on the binary pairs that make meaning, arguing that rather than being polar opposites, each was dependent on the other for meaning and (we might say) existence. (Hence one deconstructs the polarity of the binary terms.) He also showed how in all binaries, one of the terms was always subordinated to the other (man/woman, good/evil). To describe how meaning is produced, Derrida developed the term différance, meaning to differ and to defer. He focused in particular on the binary speech / writing, in which speech has been seen to provide a guarantee of subjectivity and presence in the history of philosophy and linguistics (someone has to do the speaking). Alternatively, writing is about absence, the absence of the speaker and what is signified by the written signifiers. Derrida calls the privileging of speech and presence logocentrism.

Poststructuralism rejected the theory that one could map the structure of a language or culture. Rather, meaning is constantly slipping from one sign to the next. Signifiers do not produce signifieds; they merely produce an endless chain of signifiers--hence my need to find a signifier from another semiotic system to represent the tree above. In that example, the signifier tree did not produce the signified but merely another signifier. Language works like a dictionary where, when you look up a word, you get other words that provide meaning. If you keep looking up those words, you'll ultimately come back to the word you started with.

Jacques Lacan (1901-1981) took Saussure's ideas and applied them to psychoanalysis, arguing that the unconscious is structured like a language, that is, the unconscious is a semiotic system signs stand arbitrarily for particular meanings. Lacan also postulated that every human being goes through the mirror stage in which we construct our sense of coherent selfhood by seeing ourselves in a mirror (real or imaginary; other people can also mirror us back to ourselves). But that self and its coherence are based on méconnaissance or misrecognition, because the mirror image shows us to be more unified and separate than we actually are. As in Saussure's linguistic theory, here the self has no ontology but is rather a construct, a sign, created through relationship and difference.

Michel Foucault (1926-1984) always insisted that he was not a poststructuralist critic but rather a genealogist. But his analysis of discourse owes a lot to Saussure's insights about the construction of meaning. Foucault shows how discourses regulate what can be said, what can be thought, and what is considered true or correct. So the pre-modern medical theories based on bodily humors constructed a particular understanding of the body, and within that discourse, certain things were true and false. However, there were many other propositions that were neither true nor false but fell outside the discursive system altogether. Anyone who tried to think outside the system would not have been respected or accorded a voice in the conversation about bodies. Discourse is thus the medium through which power is expressed and people and practices are governed; academic disciplines discipline. Foucault also argued that "the history of thought" is a misnomer, as it implied a continuous evoltion of ideas. Rather, he used the terms genealogy or archeology of knowledge, focusing on the ruptures or breaks between one era's discourse and another's.

Thomas Kuhn's (1922-1996) The Structure of Scientific Revolutions (1962; he wrote it as a grad student) makes the kind of argument about scientific thought that Foucault made about discourses in general (and in particular). Kuhn used the term paradigm to describe the foucauldian discourses that regulate scientific thought. For Kuhn, science is not an evolutionary, progressive march towards greater and greater truth but rather "a series of peaceful interludes punctuated by intellectually violent revolutions" (Foucault's "ruptures") in which one point of view is replaced by another. (Think of the difference between the Ptolomaic and Newtonian worlds.) So science's claim to truth is highly questionable and even ephemeral; since the truths of past science have passed away, we can be certain that what science claims today will itself one day be superseded by the claims of a new paradigm, which will itself one day be superseded . . . .

Edward Said (1935-2003) used poststructuralist ideas to analyze Orientalism, the study of the Orient by academics of the West. He showed how the academics and their disciplines constructed an object of study that had very little to do with the East (which is East, of course, only in relationship to the West, a binary relationship in which one terms has more value than the other).

The theories inspired by Saussure's linguistic theory have influenced every academic discipline because they all bear on epistemology or what can be known. If knowledge is relationship, a product of societies, the medium of power, then academic endeavor is not about the discovery of truth but rather its construction. Furthermore, the methodologies we employ in our various academic endeavors are undermined by the insights of poststructuralism. What is the relationship between the academic and the object of study? In what way can we know that object; is it available to us at all? What can we know about the past? What does it mean to interpret or analyze a work of literature? How do we choose what works to study? What is the role of the aesthetic in either art history or literary study? How is the canon of literature or art produced? How do we decide what is "good" or "beautiful"? Can there be any absolute standards of value at all if meaning is a product of arbitrary relationship and difference?

Poststructuralism has also influenced materialist theory or Marxism by providing a way of understanding ideology and showing how important it is to the maintenance of any economic system. The union of poststructuralist and materialist theory produced cultural theories and cultural studies, including, in literature, new historicism and cultural materialism, in which the goal is to understand cultures as both material and discursive. In such theories, everything can be a text (a semitic system), everything can be "read." But no one kind of text is privileged over another. All texts are literary in a sense, as they are all produced in what we might call a self-conscious manner. On the other hand, no self produces any text; there is no authorial intention; language speaks through all of us, even the most "intentional" author.

The influence of Poststructuralism, particularly in its union with materialism, is what has produced the "cultural turn" in the social sciences and humanities. And cultural criticism tends to be interdisciplinary, as the questions it asks cannot be answered from within the old disciplinary boundaries. Anyway, disciplines themselves have been called into question by the foucauldian critique of discourses. We understand them as social constructs rather than as taxonomies that arise from the nature of things.