The purpose of this article is to investigate "in situ" the functions that knowledge has when used by students in argumentative discussions. The study is based on Dewey's pragmatic perspective of knowledge, which means that knowledge gets its meaning in the activity at hand. The analyses are conducted using Transactional Argumentation Analysis, which is a combination of pragmatic meaning analysis and Toulmin's argument pattern. The empirical material consists of video-recorded lessons from two seminars in a Swedish upper secondary school. The results show that knowledge plays a crucial role in the discussions. Six different functions are identified: emphasising complexity, clarifying and correcting, highlighting conflicting interests, providing evidence in a counterargument, predicting the consequences and adding support to an earlier claim. Knowledge also has general functions, such as justifying a claim, and is part of a collective process aimed at understanding the issues discussed. Further, the students use knowledge from different disciplines, such as environmental studies, history, politics, biology and human geography.