In this talk we present a categorical proof theory for a logic for pragmatics. The aim of this logic is to provide a framework that allows to formalize reasoninig about the pragmatic force with which a sentence may be uttered. The concept of pragmatic force comes from speech act theory and plays a crucial role also in certain branches of artificial intelligence, in particular in the developement of communication protocols for software agents. Instead of considering the full-blown theory of speech acts we focus here on speech acts that either have the pragmatic force of an assertion or the pragmatic force of an obligation, and on how these speech acts can be related to each other. In particular, we are interested in a principle proposed by Bellin and Dalla Pozza that allows the propagation of obligations through causal chains of assertions. The study of such principles from the point of view of categorical proof theory is a nontrivial task and we discuss some of the issues that need to be considered in order to get soundness and completeness results.