Supporting students in making mathematical arguments is an important part of discourse practices in mathematics classrooms. Differences in teachers' support for collective argumentation have been observed and documented, and the importance of the teacher's role in supporting collective argumentation is well established. This article seeks to explain differences in teachers' support for argumentation by examining two student teachers' beliefs about mathematics, teaching, and proof to see which beliefs are visible in their support for argumentation. Assisted by a framework for argumentation and a commitment to teachers' beliefs and actions as sensible systems, we found that teachers' beliefs about the role of the teacher, particularly with respect to giving explanations, were more visible in their support for collective argumentation than other beliefs about mathematics or proof.