Starting from the “theory of rational behaviour”, in- troduced by Habermas in 1998, I attempt to demon- strate how a ective factors are entwined with those of rationality in in the decision-making processes of the mathematics teacher. This type of analysis has been carried out by developing an adaptation of the concept of “emotional orientation”, o ered by Brown and Reid in 2006. In particular, I will present the case of one teacher with her grade 9 class, involved in the explanation of linear equations.