The paper illustrates how theories from mathematics education can be applied for explaining student conception of computing science ideas. It does this by demonstrating how the theme of reducing abstraction [5] can be used for analyzing students' mental processes when the students are in the process of solving problems in Computability Theory. The topic of Computability Theory in presented here as an example for demonstrating how research in Computing Science education can use learning theories, "borrowed" from mathematics education research, for explaining learning processes.