Issues of sustainability are complex and often steeped with ethical and political questions without predefined or general answers. This paper deals with how secondary and upper secondary teachers discuss these complex issues, by analysing their aims for Education for Sustainable Development. With inspiration from discourse theory, their articulations about students as political subjects are analysed. "Critical thinking" emerged as a nodal point in teachers' discussions. In this study, critical thinking is articulated as having various qualitative meanings related to different epistemological views. On one hand, critical thinking is articulated to invite room for subjectification; but on the other hand, room for subjectification is challenged when critical thinking is articulated through the educational aims of qualification and socialisation. A consequence of changing epistemological view might be that political and ethical issues take a back seat.