In recent years, discourse in education sciences has focussed on the study of the reflective thinking of teachers. Researchers, however, do not agree on how the reflective thinking of teachers can be measured. This qualitative study looks for an answer to the question whether a cognitive map is suitable for the discovery of teachers’ reflections. The study analyses the reflections of a secondary school teacher of mathematics using an unstructured cognitive map. The sample was selected on the basis of availability. The analysis of the map was done deductively, in a concept-driven way, on the basis of the number of contacts (edges) and concepts, central, peripheral and isolated concepts and their degrees, and the levels of the map. The teacher involved was recorded commenting on the process of map-making, which was then transferred to writing. The text corpus was then analysed inductively, in a data-driven way. The results show that the teacher interpreted his own activity - which depends on pedagogical knowledge, beliefs and pedagogical situations - in a complex manner. The filter function of beliefs can be observed when the teacher’s activities and thinking did not match because some internal and external factors. The data from the map contributed to analysis of reflections and the analysis of the map added new elements to existing techniques.