People at home and trade negotiators in Geneva cannot bargain what they do not understand, and what they bargain must be based on consensual understanding among the relevant actors, whether or not they agree on what to do about it. Consensual understanding is endogenous, arising in an argumentative process of learning structured by constitutive principles of a regime. In a departure from both rationalist and constructivist approaches to negotiation analysis in political science, my goal in this paper is to try to advance analysis of these questions by exploring the contribution that deliberation or arguing makes to learning. My proposition is that something happens at the multilateral negotiation table in addition to bargaining, something that alters either the understanding of themselves and their interests that participants brought to the table, or how they understand the nature of social reality in a domain. Such learning would be endogenous to the negotiations, because it happens through interaction. This approach requires distinguishing simple learning (acquisition of new information about the context, or the preferences of others) from complex learning (new understanding of cause/effect relations in a domain), which also requires distinguishing consensual understanding from a mutual adjustment of positions. I then specify how this model might be susceptible to empirical investigation. I show how individual issues within a negotiation can be treated as cases susceptible to comparative analysis at a moment in time. I explore this possibility in a comparison of the contribution of consensual understanding to the outcome of negotiation of selected issues in the current Doha Round of multilateral trade negotiations in the World Trade Organization. I then infer the results of arguing from the textual deposits left by negotiations in order to assess the presence or absence of consensual understanding. Finally, I attempt to correlate consensual understanding with the negotiation status of the issues as of the end of the failed Doha Round ministerial of July 2008.