"As long as language is used only as a medium for transmitting information, action coordination proceed through the mutual influence that actors exert on each other in a purposive-rational manner. On the other hand, as soon as the illocutionary forces of speech acts take on an action-coordinating role, language itself supplies the primary source of social interaction. Only in this case should one speak of “communicative action.” In such action, actors in the roles of speaker and hearer attempts to negotiate interpretations of the situation at hand to harmonize their respective plans with one another through the unrestrained pursuit of illocutionary goals. Naturally, the binding energies of language can be mobilized to coordinate action plans only if the participants suspend the objectivating attitude of an observer, along with the immediate orientation to personal success, in favor of the performaitve attitude of a speaker who wants to reach an understanding with a second person about something in the world." (BFN: 18, emphasis in italics mine, in bold by Habermas).
The unique of causality of reason emerges from within only when we deliver ourselves to language itself. Language interrupts the natural causality and makes possible the emergence of the causality of reason as long as we remain delivered to the language itself.