At the heart of Jürgen Habermas's explication of communicative rationality is the contention that all speech acts oriented to understanding raise three different kinds of validity claims simultaneously: claims to truth, truthfulness, and normative rightness. This paper argues that Habermas presents exactly three distinct, logically independent arguments for his simultaneity thesis: an argument from structure; an argument from criticizability/rejectability; and an argument from understanding/reaching understanding. It is further maintained that the simultaneity thesis receives cogent support only from the Argument from (...) understanding/reaching understanding, and only if the notion of 'understanding' is expanded to that of 'agreement'. (shrink)