Die Bedeutung empirischer Theorien f\"ur die Entwicklung von   Gruppierungen
  Reviewing Piaget's psychology of reasoning by the mathematical educators of the Federal Republic of Germany late in the 1960th the concept of grouping has been understood similar to a mathematical structure. Possibly to consider empirical theories can contribute to an understanding more adequate. 
Hans Joachim Burscheid, Horst Struve
math.HO