A teacher's belief plays a significant role in the quality of teaching mathematics. In a fictive way, the researcher changed his role from a researcher to a research participant in an imaginative interview. His interior other (David) interviewed him as a researcher. A single interview session was held lasting for about three hours. The interview text was used for the analysis and interpretation using the grounded theory method. The researcher invented a substantive theory of his belief about teaching geometric transformations with Geometer's Sketchpad. The theory is "reflexive abstraction of his beliefs about teaching geometric transformations with Geometer's Sketchpad" as a personal theory characterized by some basic categories of beliefs about the advancement of pedagogy, beliefs about pedagogical environment, beliefs about the role of students and teacher, beliefs about self as a future teacher, beliefs about teaching and learning activities, and beliefs about transitions in teaching learning. The researcher reconstructed a synthesis of the characteristics of these beliefs. While constructing these layers of interpretive accounts, he used radical constructivist grounded theory as a theoretical base.