In Jacques Lacan's "The Agency of the Letter in the Unconscious; or Reason Since Freud," the author deconstructs the concept of language by relating the subject to both speech and language. His first and primary subject is the titular 'letter,' which is described as "the material support that concrete discourse borrows from language" (Lacan). The author describes the letter as a "material medium" which, though it may change with time, provides the transition between one word and another. Regardless of different spellings or pronunciations of a certain word over time, the letter is effective as long as it even roughly occupies its space within the word. The letter and its symbolic order is immutable, in that it cannot be changed in a direct sense; that change cannot really happen on a conscious level. To that end, Lacan's main point is how the letter and the symbol indicate unconscious changes, even if those changes are only existing in the immediate present.
One of the primary ways Lacan sets himself apart from philosophers like Jung is his attitude toward the unconscious and its relationship to symbols; he does not think that the unconscious strictly creates the symbols that we work with. Instead, it is necessary to look at the symbols that the unconscious produces by the letter itself. When producing, Lacan notes that there is a distinct separation between signified and signified; there is a symbolic bar that the signifier can move across, either being linked or unlinked to the signified. The signified is always static, but the signifier can often be disconnected from signified, thus creating new meanings and shifting communication dramatically. This movement creates an asymmetrical relationship that is constantly changing and never fully and completely defined.
Lacan relates this asymmetry to metonymy, the connecting of words that do not necessarily mean what they signify relates closely to our senses of desire and being. When one speaks, their signified is meant to denote the desire that is signified, but a disconnect occurs that fails to satisfy the desire. Thusly, because this kind of direct communication always fails, our desires can never be satisfied. Lacan's relation of metonymy to Freud's concept of the unconscious is quite interesting, as he rejected the idea of the chaotic, separate nature of the unconscious from the ego; he thought our speech, whether or not signifier indicated signified, was just as much of an indicator of our wants and needs as anything.
In "The Agency of the Letter of the Unconscious," Lacan makes very salient points about the disconnect between what we say and what we mean. This disconnect is often the source of metaphor and metonymy; by stating that the unconscious "is structured like a language," Lacan allows us to understand that our own id, ego and superego can be expressed and communicated, unconsciously, through language. It is through these Freudian slips and use of metaphors that we can reveal more about ourselves than we even realized. Metaphor allows our true desires to find ways toward expression, whether we realize it or not. These structures within our unconscious minds permit us to express what we want through interesting uses of signifiers to note what is not really being signified.
Lacan, Jacques. "The Agency of the Letter in the Unconscious" in The Norton Anthology of Theory and Criticism 2nd Ed. Ed. Vincent B. Leitch. New York: WW Norton, 2010. Print.