The paper explores some ways of analysing reasoning processes in the mathematics classroom and shows examples from one German classroom. It is argued that argumentation between students and the teacher differs substantially from many forms of everyday argumentation with respect to the power relations. There are situations in which students have to seek arguments for supporting assertions that are not doubtful to them because of implicit evaluation by the teacher; and the teacher usually is not in a defensive position and has the privilege of closing and evaluating argumentation. The aim of the analysis is to characterise different forms of argumentation in terms to the "authority" to which the arguments refer. It is assumed that different forms of reference are associated to different knowings.