Jacques Lacan, answered from the texts and cited to the page.
Like a language, not a language. The unconscious is not a store of instincts to be translated; it is an order of signifiers with its own laws, and Freud's dream-work had already described them without the terms.12345
This is why the place of all the dream-thoughts is marked here, on the right-hand side by this blank area where it is designated that the Ich, as such, is certainly indicated in VIII 86 each one of these dream-thoughts by finding it, by what is going to constitute what Freud calls Trauminhalt, namely, very precisely, this set of signifiers of which a dream is constituted by the different mechanisms of the…Seminar XIV, The logic of phantasy, p. 86
Iallow myself this prosopopoeia simply to make what I am saying comprehensible to those of you who have not heard it before. When I express myself by saying that the unconscious is structured like a language, I am trying to restore the true function of everything that structures under the aegis of Freud, and that in itself allows us to see our first step.My Teaching, pp. 29–30
For it is with this work that the work of Freud begins to open the royal road to the unconscious. And Freud gave us notice of this; his confidence at the time of launching this book in the early days of this century only confirms what he continued to proclaim to the end: that his whole message was at stake in this, the whole of his discovery.The Insistence of the Letter in the Unconscious, pp. 17–18
For this book inaugurates both Freud's work and his royal road to the unconscious. And we are informed of this by Freud, whose confession in letters to Fliess that have since been made public, when he launches this book toward us in the early days of this century,'¢ merely confirms what he continued to proclaim to the end: that the whole of his discovery lies in this no-holds-barred expression of his message.The Instance of the Letter in the Unconscious, or Reason Since Freud, pp. 424–425
I don't disagree, having had to pay homage somewhere here to a noble victim of the error of seeking in the letter, but I also ask how the spirit could live without the letter. The spirit's pretensions would nevertheless remain indisputable if the letter hadn't proven that it produces all its truth effects in man without the spirit having to intervene at all.Écrits, A Selection, Fink Translation, pp. 150–151
Widely adopted as a slogan, contested as linguistics, and defended by his readers as a structural claim rather than an empirical one about grammar.