For Saussure, a language is a structured system comprised of signs. A sign consists of two paired elements: the signified, or the concept associated with the sign; and the signifier, or the sound made when the sign is spoken. A sign has no meaning in and of itself; rather, it derives its meaning from all the other signs within the linguistic structure in which it participates. And the connection between any given signified and its signifier is arbitrary: the sound of the word is (with a few exceptions) unrelated to its meaning.
According to Chiesa (Subjectivity and Otherness: A Philosophical Reading of Lacan, 2007, pp. 48-49), Lacan disrupts Saussurean structuralism in three ways. First, Lacan contends that the signifier “logically precedes and causes the signified.” I suppose technically this is the case: infants make all sorts of vocalizations before they understand what these sounds mean in their language. But this seems like a trivial point. In translation, mapping words from one language to another based on how they sound is a futile undertaking. E.g., the word spelled “pain” in French has nothing to do with its phonological analog in English. Translation operates by focusing on common meaning elements, or signifiers.
Second, Lacan observes that the same signifier can have more than one signified connected to it; i.e., the same word can have multiple meanings. The complement is true also of course: the same concept can be expressed by more than one word. But what interests Lacan is that the particular meaning to which a signifier is pointing can be discerned only through context. Only after the entire sentence has been spoken is it possible for the hearer to decide with relative certainty what the speaker intended to mean by each of the words included in that sentence. In other words, it’s the structure and sequence of signifiers that determines the meaning of an utterance.
Third, the structure of the sequence of signs determines the structure of the subject who speaks them. I think what Lacan means here is that neither the listener nor the speaker knows what the subject means by what she says until after she’s said it. The words are spoken, then the meanings are assigned to the word string, then the intentionality of meaning is assigned to the speaker. Because speech takes place in a social context, the subject who speaks is embedding herself in a Symbolic order the meaning of which is already determined by the Other. Therefore, in speaking to a listener, the subject is in effect being spoken by the Other to another. The subject becomes a structural artifact of language.
It’s in Lacan’s threefold divergence from Saussure that his famous epigram “the unconscious is structured like a language” is to be understood. According to Lacan, the unconscious is made up of signifiers loosely and multiply linked to each other. In speech the unconscious strings together sequences of signifiers and routes them through consciousness, attaching them to signifieds. Signs — signifier-signified pairs — never refer to actual things in the world; they refer only to each other. But it’s in the act of pairing signifier to signified that the subject-as-unconscious is assigned meaning as a self-conscious element within the Symbolic order. The subject in effect becomes a linguistic object.
Now I’m sure there’s a whole lot to be said for looking at subjects and language in this way, but to me the whole scheme strikes me as a kind of Ptolemaic cosmology replete with epicycles and wheels within wheels that add a seemingly sophisticated complexity to phenomena that could be greatly simplified. In this case, though, the simpler understanding is also the more intuitively obvious one. People experience things; people think about the things they experience; people come up with words to describe their experiences to others. It’s not just the signifiers that are unconscious; the signifieds are too — as Donnell Stern said, “all thought is unconscious.” When we speak we consciously and spontaneously call up from the unconscious a sequence of signs in which sound and meaning are already paired up. The hearer might not know which meaning to assign to the word string until it’s been fully spoken, but the speaker can know. The speaker probably doesn’t make a radical and sequential distinction between signifier and signified; i.e., she probably doesn’t formulate a complex thought and then assign the words to those thoughts as a separate conscious act. Thought and language are more closely linked than that: we think linguistically most of the time. And the thought-word sequence is assembled in real time: the barrier between unconscious and consciousness is permeable and flexible.