Structuralism, like modern linguistics, owes a great deal to the Swiss linguist Ferdinand de Saussure. Between 1907 and 1911, Saussure gave a series of lectures that were later reconstructed from his students' notes and then published as the "Course in General Linguistics." The course outlines three principles: 1) the arbitrary nature of the sign. If the signifier "dog," for example, in English, and "chien" in French, both point to the same concept, then the link between the words and the thing signified (the animal) cannot be one and the same; 2) the distinction between "langue," or language as a system of signs, and "parole," the actual speech acts produced by that system of relations; and 3) the priority of synchronic analysis, which studies language as a system of relations unto itself, over diachronic analysis, which charts the movement of language historically through time.
For poststructuralists, Saussure posits two points of consequence: the arbitrary nature of the sign and the depiction of language as a system of oppositions based on difference. The word "cat," for instance, signifies cat because the letter "t" is not "p," as in the word "cap." The sign cat is distinguished by what it is not. Philosophers such as Derrida were quick to point out that if all signs rely on implicit traces of "differance" in order to achieve distinct meaning (as with the variability between the letters "a" and "e" in Derrida's term), then the signified concept can never truly be present in any reliable form. The belief that it can be present, Derrida contends, is "logocentric"----that is, it appeals to some foundational "logos" or ground not present in the sign itself.
It follows that we can never have "absolute" control over the meaning we intend because our intended speech is, inevitably, conditioned by an element of deferral internal to the language. What is more, since the meaning of any one term can be inscribed and re-inscribed in any number of new contexts, this "play" of the signifier, as Derrida calls it, is virtually impossible to pin down definitively. The final meaning of the text is continually deferred (or renewed), and therefore undecidable.
While a strong case can be made that much of what passes for poststructuralism is, in fact, prefigured in structuralism, a series of attacks on structuralist thought nonetheless occurred throughout the 1960s. Derrida's critique of Levi-Strauss, for instance, in a 1966 conference at Johns Hopkins University is a famous case in point.
In "Structure, Sign and Play in the Discourse of the Human Sciences," Derrida argued that the foundations of classical structuralism cannot hold without the help of some "transcendental signified" (or overarching set of principles) that the system itself works to exclude.
If our understanding of reality is invariably mediated by language and ideology, then we can, in homage to poststructuralism, entertain the following propositions: all meaning, in varying degrees, is textual and inter-textual; the author is a function of discourse rather than its source; the traditional meaning-giving author is therefore dead; discourse shapes how we come to know ourselves as subjects; discourse also determines how we encounter the world as an object of knowledge; the essence of objects is an illusion of language; and metaphysical foundations are, in essence, contingent.
Despite various depictions of deconstruction as nihilistic and immoral, poststructuralism has had a major effect on our approach to the humanities and social sciences in the latter half of the 20th century.