Can scientific research answer the 'what' question of mathematics education?

This paper problematizes the issue of how decisions about the content of mathematics education can be made. After starting with two examples where research in mathematics education resulted in different choices on the content of primary school teaching, I explore where and how, in the scientific enterprise within the domain of education, issues of the what of mathematics education are addressed. My conclusion is that, although it is often thought that scientific research cannot resolve issues related to decisions about subject matter content and goals, there is a powerful movement, with its roots in the European tradition, that pleads for the further development of didactics as a scientific discipline.