abstract: Although in the twentieth century mathematical explanation has been a somewhat neglected topic, in the past three decades interest in it revived (see Steiner 1978, Resnik and Kushner 1987, Sandborg 1998, Mancosu 1999, 2000, 2002, Mancosu and Hafner 2005, Leng 2005). In my opinion, however, all such approaches to mathematical explanation are inadequate. In this talk I will sketch an alternative approach whose basic idea essentially goes back to Descartes.